Late in the sixth century CE an otherwise unknown monk evidently in the vicinity of Amida (Diyarbakir) composed a historical work in Syriac in twelve books. While Books 1 and 2 are a miscellaneous assortment mainly of miracle stories and legendary material, the remainder--Books 3 thru 6 covering 450-491 CE, focusing on the Council of Chalcedon in 451 and its consequences in the reigns of Marcian, Leo, and Zeno; Books 7 and 8 on Anastasius and Justin I; and Books 9 thru 12, of which Book 11 is lost and Books 10 and 12 fragmentary, on Justinian and Justin II down to 569 CE--is an important witness to the ecclesiastical controversies and geopolitical upheavals of the period.
The work as a whole has been variously called either a chronicle, as here, or an ecclesiastical history. Its author proceeded for the most part by selecting, abridging, and compiling material produced by others, including narrative accounts, letters, and ecclesiastical documents originally written in Greek. Flashes of personality are apparent in occasional first-person asides, although one cannot be certain whether these are the work of the author or his sources: mild complaints about the prolixity of the Greeks (6.7b, 226); expressions of solicitude on behalf of the reader, lest he become bored by the accumulation of too much detail (e.g. 4.6d, 144); or scandalized by the retailing of salacious stories (9.19c, 369).
Throughout the work the predominating frame of reference remains the Roman East and its intractable quarrel over the mystery of the Incarnation and the relationship between the divine and human qualities predicated of the figure of Christ. Opposition to the definition of the faith reached at Chalcedon characterizes the author and his intended audience. Vivid accounts of the Sasanian siege of Amida and its aftermath and the founding of the fortress city of Dara in the reign of Anastasius (7.3-5, 232-251) anchor the work as a whole on the frontier in Mesopotamia and offer hints about the author's provenance and sources.
Yet interest abides, at the same time, in Justinian's campaign of reconquest in North Africa and Italy as in other events in the West, and the work as we have it concludes with an encyclopedic catalogue of cities and provinces attributed to Ptolemy's Geography (12.7, 431-455). Writing about the city of Rome itself with reference to the vicissitudes of Justinian's long war against the Ostrogoths, our author declares in his own voice: "God is faithful, for he will increase the later prosperity more than the first one, because of the great glory of the dominion of the Romans"--a striking vote of confidence in the future of the Roman state, as the editor remarks (10.16b, 424 with n.187), on the part of such a seemingly marginalized figure.
This anonymous, non-Chalcedonian Amidene monk is conventionally known as Pseudo-Zachariah (PZ) because authorship of the entire late sixth- century Syriac chronicle was attributed by the later chronicler Michael the Syrian and others to Zachariah (or Zacharias) Rhetor, also known as Zachariah Scholasticus or Zachariah of Mytilene, a Constantinopolitan lawyer who in the 490s published an anti- Chalcedonian ecclesiastical history in Greek covering the period from the council down to his own day. No longer extant in its original form, it attracted the notice of the sixth-century Chalcedonian church historian Evagrius Scholasticus and served as the basis of books three to six of PZ, as described above.
As the impressive list of collaborators--to whom I will refer, for the sake of convenience and with apologies for obscuring individual contributions, simply as Greatrex et al.--suggests, the present volume brings a great deal of expertise as well as painstaking care to the elucidation of a complex and composite text--so much so that the authors' detailed and lengthy introduction (1-92) itself requires a prefatory overview and finding aid (1-2).
The volume offers a translation of the critical edition of PZ published with an accompanying Latin translation in the Corpus Scriptorum Christianorum Orientalium (CSCO) series by E. W. Brooks in 1919-1924. There are prior translations into German, by K. Ahrens and G. Krüger, and into English, by F. J. Hamilton and the same E. W. Brooks, both of which happened to be published in 1899.
The Hamilton-Brooks translation (HB) is in the public domain and, as Greatrex et al. acknowledge (viii), readily available online. One routinely finds it referenced in Anglophone scholarship down to the present day,  and Greatrex et al. follow HB in omitting the mostly unhistorical material appearing in books one and two.  It is possible to wonder, therefore, whether there is a market for a new English translation and how it improves upon its predecessor.
Showing is sometimes more illustrative than telling, and so let me reproduce--without insisting upon their representativeness of the works as a whole--the two translations' respective versions of PZ's account of the enormously consequential embassy undertaken by Pope Agapetus to Constantinople in 536 (9.19d).
This western intervention had been set in motion by Ostrogothic alarm at the prospect of Justinian's imminent invasion of Italy, and it coincided with mounting concern among the Chalcedonian hierarchy in the East over solidifying non-Chalcedonian influence at court. Anthimus, recently (and uncanonically) elevated to the patriarchate of Constantinople from the see of Trebizond, was perceived to be increasingly indifferent to Chalcedon and aligned with two of the leading non-Chalcedonian figures at the time, the newly elected Patriarch of Alexandria, Theodosius, and Severus of Antioch, who had been persuaded not long before to accept an imperial invitation to the capital.
Bishop of Antioch from 512 until 518 and the subject of a Life- -written by his fellow-student Zachariah Rhetor--that is itself extant only in a Syriac translation, Severus was the leading theologian of what would become by the end of the sixth century, in spite of imperially-directed attempts at compromise, a schismatic miaphysite church. Greatrex et al. join with contemporary practice in frowning upon the term "Monophysite." In 451 the Council of Chalcedon defined the union of Christ's divine and human natures in one person or hypostasis; in contrast to this dyophysite Christology, which they were unwilling to distinguish from the Nestorianism condemned by the council, Severan miaphysites identified the divine nature in Christ with his hypostasis while professing his complete humanity and the corruptibility of his body prior to the Resurrection, distancing themselves from Eutychianism and the aphthartodocetism of Julian of Halicarnassus.
The increasingly anti-Chalcedonian tenor of the reign of Anastasius, 491-518, was met, upon Justin's accession in 518, by a pro- Chalcedonian reaction, resulting notably in Severus' deposition; Justinian struggled throughout his long reign, 527-565, to conciliate firmly Chalcedonian sentiments in the western churches while formulating a Christology that might reconcile the differences between Chalcedonians and non-Chalcedonians in the East. Some suggestion of the involuted politics of the period is offered by the fact that in 536 Zachariah Rhetor is attested, now as the bishop of Mytilene on Lesbos, participating in the non-ecumenical synod at Constantinople that condemned Severus and his followers, as a consequence of which Justinian proscribed their writings.
These developments were precipitated, PZ reports, when the Chalcedonian Patriarch of Antioch, Ephrem of Amida, dispatched emissaries who conveyed his misgivings about Anthimus to Agapetus in Rome:
[HB, pp. 267-68]
These men also accordingly came to Rome to Agapetus, and they delivered the epistle and were received; and the man was pleased with their epistle, in which he found agreement with his opinions. And he came with them to Constantinople in the month of March in the year fourteen; and Severus was there, and Anthimus was chief priest. And the whole city was disturbed at the arrival of Agapetus; and the earth with all that is upon it quaked; and the sun began to be darkened by day and the moon by night, while ocean was tumultuous with spray (?) from the 24th of March in this year till the 24th of June in the following year fifteen. And Agapetus, when he appeared before the king, had a splendid reception from him, because he spoke the same language and was chief priest of the country of Italy, which had been conquered and brought into subjection to him. And he was instructed in the outward words of Scripture but did not understand its meaning; and he held an ignoble opinion upon the Incarnation of Jesus, our Lord Christ, God the Word, and he would not consent to call the Virgin Mary the Theotokos, and divided the unity into two natures, since he held the priority of the conception of the babe, like those of the school of Diodorus and Nestorius. And he abstained from communion with Anthimus and Severus, and they yet more from communion with him; and one of them he called an adulterer and the other a Eutychianist; and he perverted the love of the king towards them and made him hostile to them; and he drove them from the city. And Anthimus and Severus and Theodosius of Alexandria made union with one another in epistles, which we have set down below; and Anthimus and Severus left the city to live each of them in hiding wherever was convenient for him.
[Greatrex et al., pp. 369-71]
These [men] arrived in Rome before Agapetus, gave him the letters, and were received. [Agapetus] was pleased with their letters, in which he found agreement with his inclination. He came with them to Constantinople in the month of March in [indiction year] fourteen, while Severus was there, and [while] Anthimus was the head of the priests. The whole city was disturbed, and the earth with all that is upon it shook at the arrival of Agapetus. The sun began to become dark at daytime, and the moon by night, while the ocean was stormy with spray from the 24th of the same month of this year until the 24th of June of the following [indiction] year fifteen. When Agapetus appeared before the emperor, he was received by him magnificently, because he [spoke] the same language and was the chief of the priests of the region of Italy, which had been conquered and subjugated to [Justinian]. He was instructed in the form of the word of Scripture, but its meaning he did not understand, and he held an injurious opinion concerning the embodiment of Jesus, our Lord, the Christ, God the Word; he did not consent to call the virgin Mary "Birthgiver of God," and he divided the unity into two natures because he maintained the priority of the conception of the infant, like those [of the opinion] of Diodore and Nestorius. He abstained from communion with Anthimus and Severus, and they even more from [communion with] him. One of them he called an adulterer and the other a Eutychian, and he changed the love of the emperor towards them and set him against them in a disputation, and [Justinian] drove them out from the city. Anthimus, Severus, and Theodosius of Alexandria became united with one another in the letter that we have copied out below, and from then on Anthimus and Severus departed from [Constantinople] to live in hiding, each one of them in the place that was suitable for him.
Substantively there is not very much that distinguishes one translation from the other, and in general Greatrex et al. seem to signal more concurrences with HB than departures. This is not at all to cast aspersions upon the meticulousness or the originality of the translators, and cumulatively their choices mark a distinct improvement upon HB. In particular, their habit of supplying the antecedents of pronouns and other clarifying details is welcome as an aid to comprehension, and close attention to the original language is paid throughout and effectively communicated to the reader.
Once the contribution represented by an exhaustive level of annotation and commentary, supplemented by maps, concordances, and appendices, is added to the account, the indispensability of the volume becomes clear. HB print seven footnotes to the passage cited above, comprising about a dozen lines of type. These identify the indicated indictional years as 536 and 537 CE, respectively (267 nn.3, 6; cf. Greatrex et al. 370 n.306 fin.); discuss the restoration of material lacking in the principal manuscript, BL Add. 17,202, from another witness, BL Add. 12,154, and Michael the Syrian (267 nn.4, 7; 268 n.2; cf. Greatrex et al. 370 n.304); credit Hamilton for his conjectural translation of the phrase "tumultuous with spray" (267 n.5, echoed by Greatrex et al.'s "stormy with spray"; cf. 370 n.305); and notice a parallel with Evagrius (268 n.1; cf. Greatrex et al. 371 n.311).
In their corresponding translation Greatrex et al. print nine notes (370-371 nn.303-311), including those already mentioned, comprising 44 lines of type occupying roughly a half of each page. Among other topics, these reference some eighteen works of modern scholarship dating from 1939; offer detailed cross-references to a range of ancient sources; reconstruct the chronology of Agapetus' embassy and assess PZ's accuracy; evaluate Justinian's motives and psychology in light of the unfolding military situation in Italy; explicate the charge of "adulterer" leveled by Agapetus against Anthimus (he had spurned his original see for another); and discuss the circulation of the correspondence establishing communion among the three anti- Chalcedonians, which PZ goes on to reproduce in chapters 20-26.
It should be clear, then, that Greatrex et al. accomplish their purpose of making PZ's work more accessible to scholars and students (viii), in no small part by distinguishing so carefully PZ's work from that of his namesake. The incredible level of detail and high standard of editing are characteristic of the Translated Texts for Historians (TTH) series more generally. 
1. E.g. Volker L. Menze, Justinian and the Marking of the Syrian Orthodox Church (Oxford: Oxford University Press, 2008), passim. As the author observes, "a thorough study of (Ps.-)Zachariah Rhetor is a desideratum" (113 n.24).
2. Greatrex et al. do however translate certain chapters in books one and two (75-92), as well as two fragmentary chapters from the later books (10.16, 12.7) that appear in neither Ahrens-Krüger nor Hamilton- Brooks.
3. I noticed very few typos for a work of such size and complexity. Let me note here that Geoffrey Greatrex has established a web page http://aix1.uottawa.ca/~greatrex/zach.html with additional notes and bibliography on the project.