The Register of Peter the Deacon, seemingly compiled in 1131–3, was not the first monastic chartulary to be drawn up in southern Italy, but was the most substantial to have been produced up to that time. Comprising 269 folios, written in double column in the local Beneventan script, it contains some 650 documents, nearly all of which were copied in extenso. Only a quarter of these, some 166 charters, are preserved in the original or in other medieval copies. Given the importance of the abbey of Montecassino, as the mother house of Benedictine monasticism and one of the major centres of the eleventh-century reform movement, and as by far the wealthiest abbey in the newly-created kingdom of Sicily, the long-awaited publication of the register provides a significant resource for the study of the Church in the region. While many of the more important documents therein had already been printed, notably by Erasmo Gattula and Luigi Tosti, archivists of the abbey in the early eighteenth and early nineteenth centuries respectively, quite a few had not, and these antiquarian editions were hardly sufficiently critical for modern standards. Furthermore this full publication enables scholars, guided by the full and extremely thorough introduction by the editors, to assess the chartulary as a whole, rather than simply using it as a source for individual documents, however useful and informative these latter might be.

The register was drawn up by a team of scribes, seemingly writing different parts at the same time, under the direction of Peter, the newly-appointed librarian and archivist of the abbey. They were apparently working in haste – as indeed they would need to have done to have completed this substantial work in slightly less than two years. The starting point would appear to have been when Peter was commissioned by Abbot Seniorectus to draw up a register of the abbey's papal bulls, or so he informs us in the introduction to the text. Subsequently, and on the advice of Prince Robert ii of Capua, this project was expanded to include other privileges granted to the monastery by secular rulers, as well as donation charters and oaths sworn by lay aristocrats (usually to respect the abbey's property). The chartulary was divided into separate sections for these four types of document. If we are to believe the story Peter told, the initial commissioning of the work must have been in 1131, shortly after he had returned to the abbey from several years in exile, as a partisan of the former abbot, Oderisius ii, who had been deposed on papal orders in 1126. The chartulary was substantially completed by the spring of 1133, as can be seen from a number of documents related to the French monastery of Glanfeuil, to authority over which Montecassino put forward (entirely spurious) claims following a visit from its abbot, Drogo, early in that year. The documents relating to Glanfeuil, some of which purported to be very ancient, were entered out of chronological sequence towards the end of the respective sections of the register. The haste with which the register was compiled can be seen in some careless transcription of documents for which other copies have survived, from mistakes in the original numbering of the charters, because a summary list was made only of the papal documents in the first section of the work and not for the other sections, and in a general lack of headings and guidance for the reader. This must, as the editors note, have made the register quite difficult to use if one wanted to consult particular documents, and suggests that in the eyes of its compiler its purpose was as much one of prestige as for the practical defence of the monastery's rights and property. Nevertheless, the latter purpose ought also to have been important, for in the previous decade Montecassino had been increasingly embattled, facing attacks on its lands, rebellion by its dependants, and a significant challenge to its independence by Pope Honorius ii.

Peter the Deacon is of course notorious as a forger of hagiographical and literary texts designed to exalt the fame and prestige of Montecassino; and indeed he has often been accused, not least by the first modern scholar to draw attention to his work, Erich Caspar, of being a deranged fantasist who invented, not merely much of the alleged early history of the abbey but also his own descent from an illustrious Roman family, the counts of Tusculum. To a considerable extent the editors rebut these charges. They follow Hartmut Hoffmann in accepting his account of his ancestry, but they also produce a convincing case that Peter was not responsible for the majority of the forgeries contained within the register. Most of the forged papal and imperial privileges were referred to in the abbey chronicle written by Leo Marsicanus c. 1090–1105, and the editors suggest that some may have been written as far back as the late tenth century. The number of outright forgeries copied in the register is anyway relatively low, less than 5 per cent of the total, with only slightly more (about 7 per cent) seeming to contain some interpolations in otherwise genuine documents.

Leo's chronicle, which made frequent references to privileges and donations granted to the abbey, was an important influence on the register. A majority of the documents copied there were ones which Leo or his continuators had already cited. Furthermore there survives independently a schedule of documents, seemingly based on the chronicle, which was clearly part of the preliminary material drawn up when planning the chartulary, for all the more than 300 charters listed there were subsequently copied in the register. Yet herein may lie a problem which the editors, in their otherwise exemplary introduction, do not raise.

It has long been known that although Peter claimed, in his introduction to book iv of the chronicle, to have written the whole of its continuation from where Leo's account ceased (after the dedication of the new abbey church in 1071), this was untrue. In fact much of the continuation, and certainly most of book iv up to 1127, was the work of a monk called Guido, who was probably Leo's successor and Peter's predecessor as librarian and archivist. Can we therefore believe Peter's claim that he was the man who drew up the register? One says ‘drew up’ because one of the few independently verifiable facts that we know about Peter is that he himself wrote only a minuscule hand, and he was apparently unable to write the Beneventan script which was still preferred for book production at the abbey in the early twelfth century. He was not therefore one of the seven scribes of the manuscript, although that does not preclude him being the overall directing intelligence. But Leo may quite possibly, c. 1100, have had some sort of schedule of documents to which he could refer when writing the chronicle, and Guido in his continuation also makes frequent references to charters. Since the last document referred to in the surviving schedule was from 1121, might not this have been the work of Guido? While the register was written in haste, is it possible that, as with the chronicle, Peter claimed the credit for work which had at least been begun by his predecessor, who would have been far more familiar with the archive than was he when he returned to the abbey in 1131? This remains only a possibility which those who study the register might consider. What is certain, however, is that all who in future study this chartulary will admire the meticulous scholarship of Jean-Marie Martin and his collaborators.