I know saying the word 'unit' probably instantly infuriated everyone, but what I'm looking for is a survey of the various types of fighter you could find in the ancient world. Equipment, tactics, the origins and development of recognizably distinct types of fighter. Partially from Alexander on to about 2 or 300 ad. Is there anything that fits the bill?
There are two, easily accessible books you can go find in pretty much the front section of Barnes and Noble that'll give you a basic introduction:
Warfare in the Classical World by John Gibson Warry and How Ancient Battles Were Fought, by an author whose name eludes me for the moment.
I also, as I always do, highly recommend anything by Adrian Goldsworthy, the man, as far as I'm concerned, is a Roman time traveller.