Ammianus Marcellinus was born between 325 and 335 A.D. and lived until at least 395. He possibly hailed from Antioch, and was of noble birth. He soldiered under Constantius II, and under Julian the Apostate until the latter's death. He later lived in the city of Rome. He wrote, in Latin, a history of the Roman empire from the accession of Nerva to the death of Valens (96-378). This work, known as res gestae, consisted of thirty-one books; of these the first thirteen are lost, the remaining eighteen cover the period from 353 to 378. Online, they can be found in Latin only, at the Latin Library. There are excerpts in translation, especially the battle descriptions: The Siege of Amida in 359 and The Battle of Hadrianopolis 378, (probably the best known). I could not find an online translation of the Battle of Strasbourg 357. And there is The Luxury of the Rich in Rome …
Ammianus was a pagan, but disliked Julian’s intolerance.
The OCD has an excellent article on Ammianus. General online coverage can be found at the The Ammianus Marcellinus Online Project, including a detailed biography. See also contributions with several essays. Related articles by Dr. Jan Willem Drijvers, the owner of project site, are all over the Internet.
Gibbon wrote that “Ammianus is an accurate and faithful guide, who composed the history of his own times without indulging the prejudices and passions which usually affect the mind of a contemporary.”
Ammianus’ work is often described as a continuation of the work of Tacitus. Leland E. Wilshire, in Did Ammianus Marcellinus Write a Continuation of Tacitus?, The Classical Journal Vol. 68, No. 3 (Feb., 1973), pp. 221-227 (JSTOR, limited access), disputes that, concluding with this:
“… we do not detract from an appreciation of Ammianus Marcellinus if we see him as part of his age and not as one consciously setting out to continue the works of Tacitus. The evidence of literary borrowings cannot be used in itself to show a dependency. The issue of Tacitus ending with the reign of Nerva and Ammianus beginning at that point is no more than a coincidence, as this was a common literary and historical turning point. The personal and philosophical outlooks of Tacitus and Ammianus show no deep affinity. The basic themes of Tacitus find no place in Ammianus. The subject matter of the Res gestae was to treat the emperors from the time of Nerva – or the fourth-century emperors in the extant material – in a biographic style patterned after Suetonius or his later imitators. It was these later imitators who were the sources for Ammianus. The importance of Ammianus as a historian is that he was able to break out of the superficiality of the “Kaiserbiographie” into an analysis, sometimes objective but always perceptive, of his age. In this analysis, he stands with his precursor Tacitus, although he had no conscious intention to write a continuation of the second-century historian. In his own right, Ammianus Marcellinus stands as the last of the great Roman historians.”
There is another JSTOR article: Ammianus Marcellinus by J. W. Mackail, The Journal of Roman Studies Vol. 10 (1920), pp. 103-118.
In print there are Loeb’s
Roman History, I, Books 14-19
Roman History, II, Books 20-26
Roman History, III, Books 27-31. Excerpta Valesiana
Penguin Classics has The Later Roman Empire: A.D. 354-378.