This could (but will not) be a short review. Robert Frost has written an outstanding book, as good as it is big--a major contribution to the history of the polity linked by the hyphen in its title, and to the history of early modern Europe. The book is a major benchmark in Frost's distinguished output addressing specific aspects of the Polish-Lithuanian Commonwealth's history, situated in the broad context of its contemporary Europe.  It consolidates Poland-Lithuania's entrance into top-tier scholarship conceived and written in English over the past twenty years or so --a process parallel, and related to, an analogous inclusion of Poland, Lithuania, and East Central Europe in the general historiography of medieval Europe. 
Frost introduces his book as an histoire événementielle, prompting my one (slight) disagreement with him. The tag is too modest and self-effacing. The book's story frames, and brilliantly develops, a wide range of subjects, all visible to the reader. This is in fact a profoundly thematic book. That said, it is indeed structured as one continuous, seamless narration, huge in its scope and its particulars. Its subjects are, so to speak, layered throughout the text, and developed in its course. As a result, the book is best read in its entirety, from beginning to end--a most worthwhile exercise, because Frost's prose is outstanding: tightly-packed yet translucent, highly engaging and interesting in basic storytelling terms, and witty. We have here a lovely example of the current return of "narrative" history at its best, into the core of what we do.
Especially conspicuous are three interrelated subjects: people, places, and constructs. The book is a biographical gallery of a huge number of individually etched actors: kings of Poland; dukes and grand dukes of Lithuania; contenders for those two offices; and a myriad other specific protagonists who made up the political worlds presided by these rulers. No less important is a collective generic actor: the political community,  above all the royal or grand-ducal "council"; the noble, or knightly, "assembly," or "general assembly"; and higher-level collectivities, such as the "nobility," "boyars," "Poles," "Lithuanians," and, perhaps most recurrently, "community of the realm." The places are: the major realms, or polities--Lithuania, Poland, Masovia, and "Royal" Prussia --of which the first two formed the principal union, the second two joined it through unions with Poland; the localities where the major phases of the story occurred, and left a written record; and the localities relevant to the governance and administration of the four polities comprising the union.
By far the most important concept treated in this book is union--in general, as a type of relationship between different realms, the Polish-Lithuanian union in particular. One of the gems of this book, a chapter titled "On Unions," previews the subject on a general level, historiographically and conceptually. The full remainder of the book can be read as a sustained development of the concepts previewed in this chapter, with reference to Poland-Lithuania. This part of the story unfolds, as markers, along those specific moments when and where the relevant individuals and communities formed, expressed, negotiated, and experienced the union, through various stages of its formation. Such places comprise the core localities noted a moment ago: Krewo, Vilnius, Radom, Horodło, Nieszawa, Mielnik, Lublin. The chapters designated with these place-names are exceptionally interesting studies of: the documents pertinent to union, created in each locality; the political ideas expressed in such documents; and, as an outcome of it all, the union, in its closely analyzed moments of existence, transformation, and ambiguity. From my perspective, its most remarkable contribution of this book is the sheer brilliance of Frost's detailed, textured analysis, at each such marker, of these twin subjects: early-modern political union in general, and Poland-Lithuania as an exceptionally interesting case study.
The book is segmented into an unusually large number of brief chapters--thirty-nine in all, ranging in length from three to twenty-two pages, most hovering around ten. They are grouped into seven parts, each treating one major chronological stage of the union's "making." Part one ("Toward Union") concerns the union's antecedents. Its five chapters include: that excellent general study of early-modern political union; historical overviews of Poland and Lithuania, extending back into the Middle Ages, with emphasis on governance and dynasty in the later fourteenth century--in Poland, the last two Piasts and the brief Přemýslid and Angevin successions, and, in Lithuania, the remarkable political structure established by the Gediminids, encompassing the Lithuanian heartland and much of post-Kievan Rus'. This part culminates with the betrothal of Jogaila and Hedwig of Anjou, articulated in 1385 at the locality variously called Krėva, Kreva, or Krewo. The resulting "Krewo act" introduces a crucial, permanent ambiguity regarding the relationship between the peoples and polities ruled by the two spouses: the exact nature of Poland's implicitly dominant position over Lithuania, and thus in the union. That ambiguity, and its implications, are the central dilemma examined in the book.
The seven chapters comprising Part two ("Establishing the Union") describe the early implementation of Krewo's terms. This part is unusually multilayered in its subjects. It continues the earlier story of the union's antecedents, now emphasizing the medieval origins of key social groups: in Poland, the nobility with its long tradition of collective action through assemblies (wiece), and, in their first appearance in the book, the peasants; in Lithuania, a society centered on the grand duke who presided over a population diverse ethnically, linguistically, and religiously, including a boyar nobility and a peasantry quite different from its Polish counterpart. A chapter on "Baptism" describes the marriage between Hedwig and Jogaila, the latter's conversion to Catholicism as Władysław, and the christianization of broader groups in the Lithuanian population--a compelling, well re-told story of the chronologically final moment in the conversion of Europe. 
These chapters open the subject of the new ruling dynasty, the Jagiellonians. After an account of its emergence from among the Gediminids, they address succession to the two highest offices, king and grand duke: the contending candidates, the dynamics of their selection, and the individuals and communities involved--including the difficult question of the roles played by actors based in each of the two realms, in succession to the highest office in the other. The chapter titled "Cousins" is a notable study of the turbulent, though ultimately stabilizing, interaction between the two originating holders of highest office, Władysław Jagiełło and Vytautas. This chapter--yet another gem in this book--contributes to the now re-emerging inquiry into collaborative rulership among close relatives in the same generation, especially brothers. 
In addition, Part two describes the union's benefits for its two partners in their long struggle against their principal adversary, the Teutonic Order. Especially welcome is Frost's placement of the military aspect into a twin context of the contemporary world, the international and the internal: Poland-Lithuania's successful diplomatic and legal navigation against the Order, soon before and at the Council of Constance; and the importance of learning at two great political communities of fifteenth-century Kraków, the court and the university. Frost does not explicitly phrase it as such, but this connection--between higher education at its best, and effective geopolitical power--is an implied lesson of this book, curiously resonant for a reviewer based in the academic world of the United States in 2016. Finally, this long part continues the book's great recurring theme, the union's formal aspects, as reflected in two more "acts" of union: in 1401, associated with Vilnius and Radom, and, twelve years later, in Horodło, especially notable for its formal adoption of a significant section of Lithuania's elite into Poland's nobility.
Consistently with its title ("Crisis, 1422-47"), Part three deals with a series of major challenges to the union. One was succession to the two highest offices in the union's two principal realms after the deaths of Jagiełło and Vitautas, especially the complexities of securing, in the political world of Kraków, succession to kingship in the Lithuanian--in a sense, foreign--dynasty; and, in Lithuania, the nearly fatal attempt by Švitrigaila and his supporters to redirect the alliance away from the union and Poland altogether. Another challenge was the prospect of Lithuanian contenders for succession in the dynasty seeking a separate royal office for Lithuania. An excellent chapter ("Rus") zooms in on Lithuania as a multiethnic and multireligious polity, and as successor to much of the post-Kievan and Orthodox world. Especially notable here is a new, post-conversion complication of rulership in the grand duchy: the limitation of eligibility for full exercise of high office to converts to Catholicism, and thus a marginalization of the Orthodox inhabitants.
The next three parts build on, and develop, the systemic resolution, or at least management, of the issues and challenges noted in the first three--effected by the wide population of individuals and communities present in both polities. Even though (with good reason) Frost marks the definitive "accomplishment," or formation, of the union a full century later (and, at the very end of the book), to this reader, the entire remainder of the book appears to shift, in its emphasis, emphasis toward a polity that, while still in continuous transition, was also now established, in terms of political ideas, practices, culture, and (perhaps) identity--or that was, to use a metaphor I rather dislike, mature. The five chapters comprising Part four ("Consolidation and Change") concern the protracted, final military struggle against the Teutonic Order, and begin the story of Royal Prussia's incorporation into the Polish Kingdom. These chapters continue the subject of the evolution of the nature of the union and of its key political constituencies--here, the definitive emergence of Poland's nobility, as a mass population and as a self-conscious political community--a process marked in 1453 by the statute of Nieszawa. This part closes with the second appearance of the peasants, now as part of the economic, legal, and jurisdictional base for that political community.
The chapters comprising Part five ("Dynasty and Citizenship") move in two directions: toward the apex of the union, as a coherent, though ever evolving polity; and toward convergence between the ruling political communities inhabiting the two major partners, as one political culture. The opening chapter ("New Monarchs") continues the story of dynastic succession to the offices of king and grand duke, presenting several especially rich biographical vignettes, including Władysław III, leader of the disastrous 1444 expedition against the Turks. The dynasty, and this king--simultaneously Polish and Hungarian--raise the subject of the Jagiellonians' international reach, and their (comparatively brief) entrance into the world of modern Europe's great dynasties--"Jagiellonian Europe."
A parallel subject is continued transformations within both realms: in Poland, the formation of its distinct form of noble parliamentary rule, centered on assemblies of regional communities, the "dietines" (sejmiki), and their central counterpart, the "diet" (Sejm); in Lithuania, continued impact of the Polish nobility upon the grand-ducal court and boyar elites, ultimately prompting the emergence of analogous local communities. Another integrating factor was the (substantial) abolition in the grand duchy of Roman Catholicism as a condition for eligibility for high office and other privileges. These transitions are well encapsulated by a chapter title bearing the Polish name for the nobility, but transliterated from the Cyrillic, as Shliakhta and by another (Litva), symbolizing Lithuania's multilingual and multiethnic profile. Finally, these chapters first observe the rise of the union's two new, permanent adversaries, Moscow, and the Ottoman Empire.
The sixth part ("Reform") concerns two watershed transitions relevant to the governance of Poland and Lithuania--specifically, in the political communities taking part in that governance, and in their relationship with the king and the grand duke. One transition was the consolidation of election of the two highest rulers by the nobility, in Lithuania and in Poland, acting as a broadly based political community. Another, made explicit by the 1505 statute Nihil Novi, was that community's far-reaching control, exercised through its parliamentary framework, over legislation--an act of strong contestation against royal or grand-ducal prerogative. The definite emergence of that parliamentary framework, in the Sejm, is a subject of an important thematic chapter ("Parliamentary Government"). Closely related is the fascinating systemic challenge in drawing into that framework the political communities of Lithuania. Similar issues are noted concerning the unions involving Poland, Royal Prussia, and Masovia. Especially interesting is Prussia: its autonomous representative framework, the communities taking part in that framework (above all, towns), and that framework's relationship with the parliamentary system of the union in its entirety.
The chapters comprising Part seven return, in a grand crescendo, to the core subject of the book: the union, its formation, and its permanent ambiguity. This is the only part of the book whose title is a declaratory statement, almost a sentence: "Union Accomplished." Like the earlier Part two, this part consists of seven chapters. In contrast, all address one subject: a definitive transformative moment in the contestation, among the political communities of Poland and Lithuania, about the nature of the union. One outcome was the recognition, by all concerned, that this was a union between two partners created "above all, equally" (aeque principaliter)--a gloriously polyvalent expression, referring to an association between partners who were essentially equal, yet with an implied qualification. Whatever that qualification means to us today, or meant to Frost's protagonists back then, by 1569 that meaning entailed a rejection, or failure, of the long-standing "Polish" claim, dating all the way back to Krewo, to a full, or a strongly understood, incorporation of Lithuania into Poland.
As narrated by these chapters, the moves by the "Polish" members of the union's political universe toward that dominance entailed several processes. One was that slightly odd phenomenon known as "execution of the laws": a sustained collective pressure, by Poland's political communities and their Lithuanian allies, toward clarification and implementation of that legislative program which had become the domain of the nobility and its representative institutions. An important part of that "execution" entailed an attempt by the "Poles" to acclaim implement their strongly incorporationist view of the union. Another was a systematic expansion, into much of Lithuania of an administrative and judicial framework patterned on the Polish. A third was the carefully orchestrated transfer away from Lithuania, and into the Polish Kingdom, of a swath territory in the grand duchy's southeast--approximately coextensive with today's Ukraine.
The complications behind these processes--including strong resistance to them in Lithuania--provoked a crisis, or "failure," of coexistence between the two partners. The resolution, described in the penultimate chapter ("Interlude"), reframed the discourse about the union, the ambiguities about its nature, the practices of its governance, and the political communities involved--above all, the Polish and Lithuania nobilities--in terms of a "commonwealth," that is, an early-modern republic. These ideas were enshrined in yet another document, issued in Lublin in 1569, the final moment in the book. Regarding that moment, Frost's final thoughts leave us with a paradox. On the one hand, he describes a major watershed in the evolution, and definitive formation, of a shared--though continuously internally diverse--political culture. On the other--literally, in the last sentence--he presents that major watershed as a reframing, but definitely not the end, of that fundamental, constitutive political ambiguity.
A notable feature of this book in its entirety is a strong authorial voice. Though never intrusive or heavy-handed, Frost conveys several messages. Perhaps most important, this early-modern union was a process rather than an outcome. From its beginnings until 1569, and implicitly thereafter, its constitutive ideas and arrangements were an exercise in ambiguity. A related message is the significance of that ambiguity. As presented here, this union was in no sense dysfunctional, intrinsically unstable, or somehow inchoate. On the contrary: throughout the long history of its "making," it was an impressive instance of early-modern statecraft, quite well equipped by the standards of contemporary Europe to confront a wide variety of challenges, domestic and geopolitical.
In sum, high-level ambiguity, in politics as in life, is presented here as a basis for a creative developmental dynamic, and as a source of strength, affecting highly pragmatic matters: governance, politics, diplomacy, and warfare. Frost conveys the same message in his treatments of individual actors. He treats them all as (at least presumptively) skilled and sophisticated political players, intrinsically deserving of a textured, respectful attention; and, in the process, he reassesses their earlier and current historiographical portraits. Among many such examples, much of the book's early portion can be read as a fresh assessment of the greatest Polish historian writing at the turn of the Middle Ages and early modernity, Jan Długosz.
Frost's third message concerns concepts and categories of analysis pertaining to union. He most forcefully argues against simplistic classification, or categorization, of unions in general, and of this one in particular. He especially resists the seductively straightforward dichotomy between "dynastic" (or "personal"), and "real," unions, and convincingly argues that that dichotomy distorts diagnosis and understanding of this union's many moments of formation, tension, crisis, near-dissolution, and survival. As an alternative, he presents a whole array of levels on which the union operated: the dynastic or personal, to be sure, but, no less importantly, the interpersonal, among that myriad of people and communities inhabiting Poland, Lithuania, Prussia, and Masovia. The result is a highly original, magnificent study of the many ways in which a political union can be "real," and a powerful argument for holding off on tidy classification, in favor of a close and textured understanding.
Frost also resolutely cautions against understanding Poland, Lithuania, Prussia, and Masovia and their ties in terms appropriate to twentieth- and twentieth-century statecraft and the nation-state. Although, with characteristic collegiality and politeness, he pulls back from tainting his earlier colleagues with "nationalism," he does identify, and thoroughly refute, the tendency, especially among Polish and Lithuanian historians writing soon after 1918, to perceive the union through a conceptual lens appropriate to the concerns of the two reemergent nation-states, and the relations between them. Instead, to the extent possible--in my view, with a complete success--he recovers the conceptual framework concerning political thought and practice as actually held by his actors, individual and collective, in the historical past. He elicits that framework from a large range of sources, discourses, and learned disciplines. Ultimately, that framework did comprise, and expressly refer to, a "state," so Frost's critique of anachronism is not an argument against statecraft as a concept--a genre long familiar among medieval historians) --but an argument for its conceptualization along lines meaningful in, and analytically appropriate to, early modernity.
In closing, I return to my one slight reservation: the self-effacing characterization of this book as narrative history. Apart from its framing of key subjects, the book it thematic on two other levels. First, some chapters comprise fascinating micro-studies on a specific subject, and so can be read independently. Apart from "On Unions" (36-46), my favorite examples are: "Cousins" (74-90), "Rus'" (158-176), "Peasants" (242-261), "Jagiellonian Europe" (277-287), "From Sejmiks to Sejm" (286-290), "Nihil Novi" (344-353), "Parliamentary Government" (354-373), "Prussia and the Union" (381-402), "Æque Principaliter" (405-423), and, closing it all, "Lublin" (477-494). Second, the book touches on, but (for good reason--so as not to become a different book) does not develop, a number of other subjects. These subjects are now distilled, so to speak, and ready for additional original research and writing.
One such subject is those big political communities: the "royal court," the noble or knightly assembly, and the "community of the realm." Another is biography. It is not surprising that Natalia Nowakowska's excellent first monograph concerns an important actor in Frost's fifteenth-century world, Cardinal Frederick the Jagiellonian.  A third is the Jagiellonian dynasty--today, a focus of a new major international inquiry, likewise coordinated by Nowakowska, at Oxford University and collaborating institutions. The multireligious theme already in the book--the conjunction of Catholicism and Orthodoxy--raises, as cognate themes, the presence and impact of the Hussites, the Reformation, and, above all, the Jews. The many stories of women protagonists inexorably take us toward gender and politics. Those are the horizons opened up by this book. Robert Frost's great achievement is to situate the Commonwealth of Lithuania and Poland at the highest level of thematic inquiry, analysis, and expository prose, fully in the company of the best work concerning comparable questions elsewhere in Europe.
1. Robert Frost, The Northern Wars: War, State and Society in Northeastern Europe, 1558-1721 (Harlow: Longman/Pearson, 2000); Frost, After the Deluge: Poland, Lithuania and the Second Northern War, 1655-1660 (Cambridge: Cambridge University Press, 2004).
2. Daniel Stone, "The Polish-Lithuanian State, 1386-1795," in A History of East Central Europe, eds. Peter F. Sugar and Donald W. Treadgold (Seattle: University of Washington Press, 2001); S.C. Rowell, Lithuania Ascending: A Pagan Empire within East-Central Europe, 1295-1345 (Cambridge Studies in Medieval Life and Thought, Fourth Series, 25; Cambridge: Cambridge University Press, 1994); Karin Friedrich, ed. Citizenship and Identity in a Multinational Commonwealth: Poland-Lithuania in Context, c. 1550-1772 (Leiden: Brill, 2009); Kristina Markman, "Between Two Worlds: A Comparative Study of the Representations of Pagan Lithuania in the Chronicles of the Teutonic Order and Rus'," Ph.D. diss., UCLA, 2015.
3. For surveys of the literature, see, in its entirety, Nora Berend, Przemysław Urbańczyk, and Przemysław Wiszewski, Central Europe in the Middle Ages: Bohemia, Hungary and Poland, c. 900–c. 1300 (Cambridge: Cambridge University Press, 2013); and Piotr Górecki, "Piast Poland and the Legal Systems of Medieval Europe: A Case Study," Quaestiones Medii Aevi Novae 20 (2015): 5-34, at 5-8, nn. 2-15.
4. Developed in an excellent adaptation of Susan Reynolds's classic study of that subject, Kingdoms and Communities in Western Europe, 900–1300 (2nd ed.; Oxford: Oxford University Press, 1997).
5. On the latter, see Karin Friedrich, The Other Prussia: Prussia, Poland and Liberty, 1569-1772 (Cambridge: Cambridge University Press, 2000, repr. 2006), translated into Polish as Inne Prusy. Prusy Królewskie i Polska między wolnością a wolnościami (1569-1772), trans. Grażyna Waluga. Poznań: Poznańskie Towarzystwo Przyjaciół Nauk, 2005.
6. For earlier stages of this process in nearby regions, see Nora Berend, ed. Christianization and the Rise of Christian Monarchy: Scandinavia, Central Europe and Rus', c. 900-1200 (Cambridge: Cambridge University Press, 2010).
7. Jonathan Lyon, Princely Brothers and Sisters: The Sibling Bond in German Politics, 1100-1250 (Ithaca: Cornell University Press, 2013); Aneta Pieniądz, Więzi braterskie we wczesnym średniowieczu. Wyobrażenia i praktyka społeczna (Kraków: Homini/Tyniec Wydawnictwo Benedyktynów, 2014).
8. A most convincing response to, and departure from, that genre, is Susan Reynolds, "The Historiography of the Medieval State," in Companion to Historiography, ed. Michael Bentley (London: Routledge, 1997), 117-138.
9. Natalia Nowakowska, Church, State and Society in Renaissance Poland: The Career of Cardinal Fryderyk Jagiellon (1468-1503) (Aldershot: Ashgate, 2007; repr. Abington: Routledge, 2007), translated into Polish as Królewski Kardynał. Studium z kariery Fryderyka Jagiellończyka (1468-1503), trans. Tomasz Gromelski (Kraków: Societas Vistulana, 2011).