Gaul is a whole divided into three parts..........
I think that Caesar's Gallic Commentaries is extremely informative, it gives us a detailed descriptions of the geography and of the inhabitants of not only Gaul and it's many tribes like the Aedui, Helvetti, Belgae, Veneti, Suebi, but also the Germanic tribes aswell as a very detailed description of the British expedition.
Caesar gives an indepth analysis of the workings of the many different tribes and their beliefs, he tells us how they lived, how they dressed, what they ate, how they formed for battle, he also informs us of the workings of the Roman army, battle formations, siege works, defending camps, soldiers morale, how they shared up the booty, keeping the supply lines flowing etc.
Yes i'm sure some of the casualties of battle and the resounding Roman victories are a slight exaggeration of the truth but what the hell, i'm pretty sure he's not the first author and definately not the last to strech the truth a bit.