The Res Gestae Divi Augustae or The Deeds of the Divine Augustans is a work of Roman history written by one Ammianus Marcellinus.
It begins where Tacitus had left off in 96 AD an continues until the battle of Adrianople in 378 AD, and consisted originally of 31 books in total, although only books 14 to 31 (which cover the years 353 to 378) have survived. Generally considered to be reliable and impartial history, if only because Ammianus was a contemporary witness to many of the events described therein.
One of the prime historical sources for the fourth century AD.
There is a copy of the Latin text of the Res Gestae at http://www.geocities.com/Athens/Forum/6946/literature/ammianus.html. But I have been unable to find an online English translation.