Res Gestae by Ammianus Marcellinus

Good account of a soldiers life (Ammianus) which contains details of the time he served under Julian in Gaul and Persia. It is pro-Julian in sentiment, but gives a good impression of why Julian was popular with soldiers for his effective command and handling of men. Goes on to detail his reign as Emporer and actions taken during this time. Number 871.9 in the information commons, only books 14-26 survive, so dont spend ages looking for the first ones.