Rennie's scholarship to date has focused on aspects of papal legation in the eleventh and twelfth centuries.  The Foundations of Medieval Papal Legation steps back in time to trace the development of the office from its beginnings through the mid-eleventh century.
Papal legation was a cog in the wheel of the papacy's expanding government and thus the need for representation grew in step (2, 3). Individual popes, contemporary legal needs, and socio-political ambitions all contributed to the growth of the office. Popes assigned legates on an ad hoc basis to respond to immediate needs and circumstances (16-17). Centralization of the papacy brought overtures of subjection and legates played an important role in establishing/enforcing bonds of obedience (8). They efficiently and effectively connected the Roman center with the Christian periphery. Early versions of legation, according to Rennie, were not rooted in the same legal traditions as those after the 1070s (5, 6). The framework developed by the end of the ninth century was realized under church reformers, the reason being the need for greater jurisdictional and juridical freedom of representation (169). The work poses a series of questions: "why it was necessary to extend papal influence beyond Rome and its surroundings in the first place; what were the benefits and consequences of so doing?; what were the social, political, legal, administrative, and institutional conditions impelling this development?; and, considering these and other questions in the larger context of a growing Latinized and Christianized world in the European West c.300-1000, how did papal legation impact the growth of Christianity and its respective western kingdoms" (6-7)?
Following chapter 1 (which lays out the aims, arguments, and suggestions posed in the work as well as the historiography comprising of institutional, national, and pontifical histories), Rennie analyzes the notion of representation through the lens of Max Weber's sociological theories. Representation, which entails whether the appointment is permanent or for a limited time and whether the assignment pertains to a member or to outside persons, blended what Weber called "instructive" and "free" representation. "Instructive" representation limited one's powers by mandate and preserved the right to recall the representative. "Free" representation afforded more freedom of movement and privileges (21). Representation, however, hinges on the legitimacy of authority. Weber's theory of "legitimate order" helps to conceptualize the legate's overall function as a symbol of primacy. Bureaucratic and patriarchal structures fostered stable relationships among the community of Christian followers. Rational belief in tradition and the legality ascribed to the Roman Church and to the papacy provided a committed measure of obedience. The legate's coercive powers--which reinforced the primacy of the Roman Church as the source of legitimate order--were manifested along rational, traditional, and charismatic lines, which Weber called the "three pure types of legitimate authority" (26-28). Bureaucracy and law were important prerequisites for the creation of legitimate order. The embassies of classical Greece and Emperor Constantine I's efforts to improve imperial communications in the Roman Empire provided implicit models for medieval legation (37-39). Both illustrate Weber's theory that representation and responsibility constitute a "governing authority," whose basis of independent (i.e. legitimate) power was determined and bound by corporate identities, rules and procedures. Both the Digest (1.16, 1.19, 1.21, 2.1) and Codex (2.12.10) address the representative role of procurators and thus mirror what Weber called the "specified sphere of competence" (29-32).
Having laid a theoretical foundation for representation, Rennie relies on conciliar and papal sources and traces the development of the office chronologically. From the fifth century, the apostolicus vicarius--an office that resonates with that of the legatus natus--provided strategic contacts for governing the Christian provinces from a distance. The vicars (i.e., deputies of praetorian prefects) used during the imperial tenures of Diocletian and Julian served as a model. As the pope's representative in an ecclesiastical jurisdiction, the duties of the apostolic vicar were almost synonymous with those of a metropolitan. They served as the eyes and ears of the pope and aided in tying the Christian periphery to the papacy. The earliest apostolic vicar was the bishop of Thessalonica (Illyricum) appointed in 380 by Pope Damasus followed by the bishop of Arles in 514 under Pope Symmachus (41-55). The papal apocrisiarius--the point-person between Rome (Church) and Constantinople (State)--originated with Bishop John of Cos's commission by Pope Leo I in the middle of the fifth century. Pope Gregory I elaborated on the office. The apocrisiarius relayed messages on behalf of the imperial chancery; operated as a military judge; handled religious matters; was sent as an envoy from the patriarchal sees of Antioch, Jerusalem, or Alexandria; and maintained political communications between the Byzantine emperor and Rome. The nature of the office meant that the apocrisiarius had to follow papal mandates, exercising powers and issuing rulings designated by individual popes. In civic affairs, by contrast, their activity was collectively determined by imperial law (55-64). Both the apostolicus vicarius and apocrisiarius fell into disuse as legatine offices between the mid-eighth and ninth centuries on account of the Frankish-papal alliance and the augmentation of papal power (though Rennie notes that in Gregory I's pontificate the metropolitan see of Corinth was already competing with the bishop of Thessalonica). The apocrisiarius ceased to operate between Rome and Constantinople both because papal claims to spiritual supremacy were developed and extended to claims to temporal authority and possible because of the Iconoclasm Controversy.
The more flexible office of papal legatus ad causam operated alongside and eventually replaced the institutionally and jurisdictionally limited offices of apostolicus vicarius and apocrisiarius. Rennie would isolate the period of the mid-eighth through the ninth century as a pivotal turning point for the history of legation because of the burgeoning papal administration and to the socio-political realignments involving the Frankish kingdom (north-south axis) and Byzantium (east-west axis). A "blanket term," legatus ad causam comprised of the nuncius, defensor ecclesiae, and legati vagantes (66). The nuncius was a messenger of written and oral communications. He announced, declared, reported, related, narrated, made known, informed, and gave intelligence and could do little more than express the will of his master; he held no powers to debate, settle, or conclude (67-72).  The defensor ecclesiae evolved from an older civic administrative counterpart (defensor civitatis). Chosen from the ranks of lector, sub-deacon, deacon, and priest, the defensores ecclesiae existed since the fourth century as protectors of papal patrimonies in Gaul, Africa, and throughout the Italian peninsula as well as protectors of the poor. By the late sixth and turn of the seventh century, defensores ecclesiae formed a diplomatic corps acting as the pope's eyes and ears by carrying out various administrative functions (72-79). Finally is a category of Rennie's own creation: the legati vagantes--"wandering legates"--who were missionaries equipped with the essential skills of persuasion and negotiation. They were often dispatched for long periods of time to both convert and to correct the Catholic faith. Oftentimes, however, missionary and legatine activities merged, such as in Bulgaria and Scandinavia (79-87).
Rennie addresses who became a legate in chapter 5 and how he exercised his powers in chapters 6 and 7. Apostolic vicars were chosen from episcopal ranks for reasons of familiarity with the ecclesiastical politics, customs and traditions of their assigned region/jurisdiction. Furthermore, they were non-Roman, as seen in Thessalonica and Arles, because of the need for local knowledge. Deacons were appointed as apocrisiarius as they had the advantage of mobility. Unlike the parish priest or bishop, they did not have a church and thus they were available for long term appointments. The defensores ecclesiae (a legati ad causam) were chosen from the episcopal rank, owing primarily to their close relationship with the pope. However, Rennie noted that Pope Gregory I also chose subdeacons, and deacons to hold this position (90). Examples of tramontane legates notwithstanding, Rennie concludes that familiarity with the pope, that the legate was from Rome (i.e., cisalpine), and that he was a company man were all paramount (94). While legates primarily came from the episcopal rank because their prestige carried the weight necessary to achieve a mission's objectives, in reality they came from a variety of ecclesiastical ranks. Bishop-legates convened and presided over councils. But Rennie also gave the example of Pope Hadrian I who sent the cubicularium Anastasius to Charlemagne seeking help against the recalcitrant archbishop of Ravenna (99). He also noted that during the mid-eighth century dealings with Frankish kings gave witness to the varied commissioning of exorcists, archdeacons, deacons, priests, abbots, bishops, archbishops, notarii (notaries), mansionarii (church wardens), cubicularii (chamberlains), sacellarii (treasurers), primicerii and nomenculatores (dignitaries of the papal court), defensores ecclesiae (defenders of the Church), viri religiosi (religious men), and bibliothecarii (librarians) (94).
Commonitoria ("credence letters"/"mandates") outlining the expected duties and responsibilities commissioned to individual legates are sparse in the first four centuries (106), and, while some fifth century examples explain the powers of legation in detail, Rennie notes that "[a]ll too often the intricacies of legatine introduction, jurisdiction, and power are glossed over as taken for granted" (110). The letters served to remind the addressee of the authority from which the specific legation originated. On occasion, they translated into a broader explanation of ecclesiastical duties and responsibilities (108). One such duty was to convene and to preside over councils, which helped to expedite local and regional cases and to prevent trivial matters from flooding Rome (123). The legate was an arbiter and judge on behalf of St. Peter (153), and he played a role in defining orthodoxy by addressing doctrinal matters. Under Pope Leo I, the office of papal legate had obtained a degree of specialization in both administrative and legal terms, with an emerging classification system of temporary legates, vicars, and permanent legates (128). Provincial synods from the 4th to 10th centuries showed the true range of powers, privileges, duties, and responsibilities. Rennie used the Council of Sardica as a case study of how the papacy used legates to connect Rome with periphery (134-135). Legations, however, were not always successful as shown by the case studies of the Council of Carthage in 419, the Council of Constantinople in 861, and the Council of Metz in 863 (138-140, 144-150).
Rennie's ambitious project covers sociological theory and approximately seven hundred years of legatine history in 175 pages. This is no easy feat, as he has had to grapple with the nature and quantity of source material. He admits, on at least three occasions, that the paucity of evidence (89) or the piecemeal nature of the sources (90-91) or the fragmentary evidence (123) made exploring the topics more challenging. Covering such a vast amount of material in a limited spaced does pose some issues with organization. For example, chapter 2's analysis of sociological theory could have benefited from a recapping and interweaving in the conclusion. How does theory help us to understand what has been analyzed throughout the work? Rennie opens chapter 4 with the note that the legatus ad causam is "often perceived as the prototype for the later offices of legatus natus (native legate), legatus missus (legate who is sent), and legatus e/a latere (legate from the side)--offices which came into play between the Carolingian and church-reforming eras (ninth to eleventh centuries)" (65). However, he does not really address in detail the shift from the former to the latter, even though he noted that the legati ad causam were still active by the eleventh century (86). Rather there is a transition to Pope Zacharias's appointment of Saint Boniface as the legatus apostolicae sedis--essentially a legatus natus--to Francia in 748 (156), the importance of the pallium as a tangible link to Rome (161), and the roots of the legati e/a latere in the Injunction of Jeremiah (163-165, esp. 164). While one could see how the nuncius was the antecedent to missus, what was the relationship between the defensores ecclesiae and the legatus e/a latere; and what of the legati vagantes? While chapters 3 and 4 deal with types of legates, the socio-political factors impacting these offices are analyzed in chapter 8. The intervening chapters, chapters 5 through 7, at times blend together the distinctions made in the previous chapters and are not situated clearly amid the socio-political changes of the following chapter. Did it matter which type of legate represented the papacy at the various councils? Do the sources allow us to delve deeper into the role played by the socio-political realignments of the eighth and ninth century and who became a legate? While Rennie noted that the commonitoria of Pope Gregory I are very helpful for outlining operational matters--settling disputes and convening his fellow bishops, but being limited in matters of doctrine or faith (112-113)--how did commonitoria change over time, particularly in the mid-eighth through ninth centuries? Did legatine duties change and evolve or was it simply a matter of the titles changing along with the associated flexibility? Compounding matters was the editorial choice to use endnotes as opposed to footnotes, which makes cross-referencing the text with the notes frustrating.
My small quibbles with organization stem from the chronological approach of the work. The aims, arguments, and suggestions appear in ten separate places (eight of which are in chapter 1).  While none of these renditions contradicts one another, they also do not come together into a simply stated thesis. Tracing the historical development of the legatine office has led to overlap in some places and fails to connect ideas in others. I personally found Rennie's observation that "the archetypal medieval papal legate was born in the mid-eighth century, as a direct result of altering political ties from an east-west to north-south axis" (157) very interesting and thought that it would have anchored the work well. Beginning with the importance of the changing socio-political climate with regards to the Frankish alliance and Byzantium in the eighth to ninth century could have provided a sturdy platform to analyze the legati ad causam and then step back to analyze the history of early legation (i.e., the offices of the apostolic vicar and apocrisiarius), on the one hand, and how legation continued to develop through the eleventh century (i.e., the transition from the different legati ad causam to the legati natus, missus, and e/a latere), on the other. The content of chapters 5 through 7 could be woven into the relevant section and would provide a compact analysis of all aspects pertaining to that office in one location. Such an organization might have also reduced the problematic nature of the sources by simply showing how early medieval legation differed from the 1070s forward rather than having to prove how it developed.
Despite some nitpickings, Rennie's work provides an introduction to the field of papal legation, a field often studied from the twelfth century forward. It raises a number of interesting questions for further study and has opened the door to the connection between the growth of the office and the growth of the papacy, which happened in step with one another.
1. See "The Injunction of Jeremiah: Papal Politicking and Power in the Middle Ages," Journal of Medieval History 40 (2014): 108-122; Law and Practice in the Age of Reform: The Legatine Work of Hugh of Die (1073-1106) (Turnhout: Brepols, 2010); "At Arm's Length: Papal Legates in Normandy (11th and 12th centuries)," Revue d'histoire ecclésiastique 105 (2010): 331-345; "The Council of Poitiers (1078) and Some Legal Considerations," Bulletin of Medieval Canon Law 27 (2008): 1-20; "Hugh of Die and the Legatine Office under Gregory VII: On the Effects of a Waning Administration," Revue d'histoire ecclésiastique 103 (2008): 27-49; "Imbutus divinis dogmatibus: Some Remarks on the Legal Training of Gregorian Legates," Revue historique de droit français et étranger 85 (2007): 301-313; "'Uproot and Destroy, Build and Plant': Legatine Authority under Pope Gregory VII (1073-85)," Journal of Medieval History 33 (2007): 166-180; "Collaboration and Council Criteria in the Age of Reform: Legatine Councils Under Gregory VII," Annuarium Historiae Conciliorum 38/1 (2006): 95-118.
2. Rennie removes nuncius from the umbrella of legatus ad causam when he notes that: "Whether commissioned with full or specific papal powers, acting as nuncii, vicarii, apocrisiarii, or legati ad causam, their mere presence held sway in the political and ecclesiastical arena" (88).
3. Pp. 2, 3, 5 (twice), 6, 8 (twice), 16-17, 169, 174.