This was discussed at some length here.

I would just add that although people in that thread recommend Stephen Dando-Collins's book about Legio X Equestris, which was quite loyal to Caesar, he makes some fundamental errors, such as getting confused by the fact that there were multiple "tenth legions," and this leads him to credit them with a few actions incorrectly. It's eminently readable, however. I read it in my free time way back when I was a freshman undergrad.

I recommend the Smithsonian History of Warfare series's text on Roman Warfare for a good survey of how Roman armies operated.