I think Kate Gilliver's Caesar's Gallic Wars, 58-50 B.C. is a good place to start. Gilliver is a Roman military historian and archaeologist based in Cardiff, and is more than qualified to write on the history of the Gallic Wars. The book itself is more about the war than Caesar, and there is even a subsection in one chapter that provides a portrait of one of the centurions in the Roman army, P. Sextius Baculus.

I have seen this book listed for exorbitant prices online (Routledge lists it for more than $100), but the eBook (at least in the USA) is less than $15 on Amazon, which I have linked above.