The first half of the book talks about the general form of battle tactics from about 500BC to 50BC. This part is good as an introduction to common tactical themes like outflanking the enemy, dissimulation, etc. In parts the descriptions get too bogged down in names and places which makes it difficult to understand the gist of the battles.
The second half of the book goes into detail on something like 12 different battles of that same era. This is the real meat of the book, and is written very well. The author gives battle plans and diagrams, walkthroughs of the battles, and tactical analysis of what went right and what went wrong.