Peter Brown details circumstances behind the series of Augustine’s works found in 1975 (the Divjak Letters) and the series of Augustine’s sermons found in 1990 (the Dolbeau Sermons):
In 1975, Johannes Divjak of Vienna (on mission from the Austrian Academy, to catalogue all manuscripts of Augustine in European libraries) found a mid-fifteenth century manuscript in the Bibliothèque Municipale of Marseilles. Produced in around 1440 for King René of Anjou, a rich but unfortunate monarch, the author of a courtly novel in the best late medieval manner, The Story of a Heart Caught by Love, the manuscript had been known, but had not been closely examined. It was assumed that an elegant late medieval manuscript could hardly contain any new work of an author as frequently copied as was Augustine. Hence the surprise of Johannes Divjak when, on examining the text, he found that it contained, added to a standard collection of Augustine’s letters, twenty nine other letters, of which twenty seven (many of them very long) were utterly unknown. Known now as the Divjak Letters, these twenty nine letters tell us in great detail about hitherto unknown events and about the activities of Augustine as a bishop in Roman North Africa in the last decades of his life: the longest and most vivid of them range from between 419 and 428.
Yet again, in 1990, François Dolbeau perceived that an apparently uninteresting, badly-copied manuscript of the late fifteenth century, recently catalogued in the Stadtbibliothek of Mainz, contained groups of sermons known previously only through titles in Possidius’ Indiculum and through Carolingian library lists of sermons and a few, short extracts. They were first announced to the learned world as the Mayence Sermons (from the French word for Mainz, the place of their discovery) and are now known as the Dolbeau Sermons, from their discoverer. One cluster of these sermons represents Augustine’s preaching at Carthage in the spring and summer of 397—that is, in the crucial year of the beginning of his career as a bishop, at a time when the Confessions were already forming in his mind. The other group of sermons takes us to Carthage and the little towns outside Carthage in the late winter and spring of 403-404, at a time of urgent reform in Catholic worship combined with new Catholic aggression against pagans and Donatists.
Augustine against the Donatists is interesting. :-)