Short answers, pulled from the story.
Ammianus Marcellinus was a Greek-born Roman soldier and historian who lived from around 330 to somewhere between 392 and 400. He served in the armies of emperors Constantius II and Julian before settling in Rome and writing the Res gestae, a history of Rome from 96 to 378.
Ammianus wrote the Res gestae, originally thirty-one books covering Roman history from the accession of Emperor Nerva in 96 to the death of Valens at the Battle of Adrianople in 378. The first thirteen books have been lost; the surviving eighteen books cover the years 353 to 378.
Ammianus was born around 330 in the East Mediterranean, possibly in Syria or Phoenicia, into a noble family of Greek origin. His native language was Greek, but he also knew Latin. He described himself as "miles quondam et graecus" - a former soldier and a Greek.
Ammianus served under Ursicinus in Mesopotamia and accompanied him on campaigns in the east against the Persian forces of King Shapur II in 359. He also served in Gaul under Julian, participated in campaigns against the Alamanni and the Sassanids, and narrowly escaped the Persian sack of Amida.
Almost all surviving copies of the Res gestae trace back to a single ninth-century Carolingian manuscript, Vatican lat. 1873, made at Fulda. The only independent source is the Fragmenta Marbugensia, a second ninth-century codex dismantled in the fifteenth century for bookbinding, of which only six leaves survive. In 1936 R. P. Robinson demonstrated that the Vatican manuscript was copied from the Frankish one.
Gibbon called Ammianus "an accurate and faithful guide" who wrote about his own times without the usual prejudices of a contemporary. He also criticized him, writing that "the coarse and undistinguishing pencil of Ammianus has delineated his bloody figures with tedious and disgusting accuracy."