Warfare in the Classical World by John Warry is a good, solid introduction to the whole period and all/most the major powers in just one book. Next I'd go for Peter Connolly's Greece and Rome at War. Then, there are scores of more specialized titles. But if you want just one book, which is nicely illustrated, readable and that will get you acquainted with the basics of warfare in the period of RTW, then you can hardly go wrong with Warry's piece.

Cheers
A.