Ammianus Marcellinus 
The Roman History of Ammianus Marcellinus [EPUB ebook] 

Covering the reigns of the emperors Constantius, Julian, Jovianus, Valentinian, and Valens. According to Wikipedia: ‘ Ammianus Marcellinus (325/330–after 391) was a fourth-century Roman historian. He wrote the penultimate major historical account surviving from Antiquity (the last was written by Procopius). His work chronicled in Latin the history of Rome from 96 to 378, although only the sections covering the period 353–378 are extant.’