If it's military history you are interested in, Osprey has published some decent handbooks in a very accessible format, for example 'Imperal Roman Legionary', 'The Roman Army of the Principate 27BC-AD117', 'Roman Army from Caesar to Trajan', 'Roman Republican Legionary 298-105BC'.
A treatise I found only recently and was very impressed with was 'Arms and Armour of the Imperial Roman Soldier: From Marius to Commodus' by G. Sumner and Raffaele D'Amato, from 2009, which to me at least offered a lot of new insight.
All of these can easily be found online.