Over thirty years ago Graham Webster wrote The Roman Imperial Army. It used the forensic approach, with each chapter looking at the soldiers uniforms, daily exsistence, equipment, organization, leadership, training, installations, and of curse how the Roman Army fought it's many wars. Webster's book has since become the touchstone for this area. Some fifteen years ago Dr. Lawrence Keppie wrote The Making Of the Roman Army. Taking the political viewpoint he showed the evolution of the Roman Army from the days of the late Republic to the early Empire. together the two books make a fine set.
The Roman Cavalry completes it. Long overlooked by historians thanks to only sketchy information from Roman writers ( the Romans were not horse people ) this work sets out to fill in some gaps. Relying not only on Roman works, but cavalry works from before the Roman Empire and as recently as the nineteenth century the authors manage to fill in those areas which until now have remained blank. The writers also rely on recent archeological finds that have helped to broaden our knowledge of the Roman cavalry. Not surprisingly they follow Graham Webster's approach and each chapter deals with a certain aspect of the horse soldier. Their equipment and weapons, training and recruitment, daily living, tactics and battle, religion, etc. Handsomely illustrated the book is very readable, never lagging, and well researched. A very admirable effort considering the lack of research material.
Excellent source of detail on what was, for the Romans, a more niche arm of the army for most of their history. Covers recruitment, training, organization, training &c. A great supplement to the more usual books covering the legions and auxilia.