About 385 Roman historian, Ammianus Marcellinus, wrote Res gestae libri XXI, the last major surviving historical account of the late Roman empire. His work chronicled the history of Rome from 96 to 378, although only the sections covering the period 353 - 378 remain extant.
“This is the history of events from the reign of the emperor Nerva to the death of Valens, which I, a former soldier and a Greek (miles quondam et Graecus), have composed to the best of my ability. It claims to be the truth, which I have never ventured to pervert either by silence or a lie.” (Amm. Marc. 31.16.9)
“An accurate and faithful guide, who has composed the history of his own times without indulging the prejudices and passions which usually affect the mind of a contemporary.” (Edward Gibbon)
The above quotations are from the introduction to the Ammianus Marcellinus Online Project directed by Jan Willem Drijvers of the Rijksuniversiteit Groningen. This site concerns the biography, bibliography, editions, translations, commentaries, concordances, etc. of Ammianus Marcellinus.
"His [Ammianus's] work has suffered terribly from the manuscript transmission. Aside from the loss of the first thirteen books, the remaining eighteen are in many places corrupt and lacunose. The sole surviving manuscript from which almost every other is derived is a ninth-century Carolingian text, Vatican lat. 1873 (V), produced in Fulda from an insular exemplar. The only independent textual source for Ammianus lies in Fragmenta Marbugensia (M), another ninth-century Frankish codex which was taken apart to provide covers for account-books during the fifteenth century. Only six leaves of M survive; however, before this manuscript was dismantled the Abbot of Hersfeld lent the manuscript to Sigismund Gelenius, who used it in preparing the text of the second Froben edition (G). The dates and relationship of V and M were long disputed until 1936 when R. P. Robinson demonstrated persuasively that V was copied from M. As L.D. Reynolds summarizes, 'M is thus a fragment of the archetype; symptoms of an insular pre-archetype are evident.'
"His handling from his earliest printers was little better. The editio princeps was printed in 1474 in Rome by Georg Sachsel and Bartholomaeus Golsch from 'the worst of the recentiores', which broke off at the end of Book 26. The next edition (Bologna, 1517) suffered from its editor's 'monstrously bad conjectures' upon the poor text of the 1474 edition; the 1474 edition was pirated for the first Froben edition (Basle, 1518). It wasn't until 1533 that the last five books of Ammianus' history was put into print by Silvanus Otmar and edited by Mariangelus Accurius. The first modern edition was produced by C.U. Clark (Berlin, 1910-1913). The first English translations were by Philemon Holland in 1609, and later by C.D. Yonge in 1862" (Wikipedia article on Ammianus Marcellinus, accessed 12-29-2013).
The editio princeps of Ammianus Marcellinus, Historia, libri XIV-XXVI, edited by Angelus Sabinus, was issued in Rome by printers Georgius Sachsel and Bartholmaeus Golsch on June 7, 1474. ISTC. No. ia00564000. This was the only printed edition published in the 15th century. In February 2014 a digital facsimile of the editio princeps was available from the Bayerische Staatsbibliothek at this link. Perhaps the lack of acceptable manuscripts prevented scholars from attempting better printed editions of the text until 1533.