The Gallic War

If you haven't read it, it's a pretty good read. It's unlike the other most recent historical book I've read, Herodotus' Histories, in that it's a pretty easy read and there's not nearly as much misinformation, but that's probably because Caesar (or his ghostwriter, depending on your personal inclinations) lived in a time much more travelled and catalogued.

It's a great book, and the details on natives and their flora and fauna is interesting and, at times, fairly detailed, such as the note that elks have no joints in their legs, and grisly details on druids and wicker men. It's really about everything you need to know to run a fairly successful low-magic campaign in the area. Of course, you could add goblins and dwarves and

The point I was going to make, though, is that you can flip open the book, and immediately find a good adventure hook. Some of the things I can remember off the top of my head:

• Caesar advances into Gallic territory, but in so doing, he crosses a river. His soldiers build a bridge across, and in so doing, station a legion at the tower. This tower is on the border of Gaul and "Germany" (really, a collection of Germanic tribes). You and your party are the captains of this legion.
• Caesar's scouts use a blitzkreig style cavalry rush to find a traitor. You find him with orders to capture him and bring him to Caesar, but he escapes in the commotion and is now hiding amongst the common people. You must find him, and quickly, before he gets away.
• Caesar leaves for the winter, and leaves your legion as a garrison in a rebellious enemy's territory. You must convince them to put down hopes for their rebellion or, failing that, must endure a seige from the natives in unfamiliar territory. 
• You are one of Caesar's cavalrymen, drawn from the native countryside. Your countrymen recognize you in battle, and call you names. Caesar uses you to infiltrate the enemy camp to gather intelligence. Do you stay with your fellows, or do you return your loyalty to Rome?

That sort of thing. The situations Caesar and his legions find themselves in are always interesting, and usually dangerous, and even if you're not into the military campaign style of gaming, you could always be treasure-hunters, mercenaries, or just power players in the giant vacuum that Rome accidentally creates when they kill the native leaders and make the next in line swear fealty to Rome. What do you do now that you could have it all?