Hmmm... One book that most fired my imagination about gaming worlds: Jared Diamond's Guns, Germs and Steel. In this book, the author gives an explanation (worthy of an introductory sociology class, so don't expect graduate-school level reading) as to why Europeans went out and conquered the world, and not the other way around.

He quickly dismisses the standard racist B.S., stating that Europeans aren't in any way better than Africans, Asians, or Native Americans. It's all a matter of the luck of the geographical draw.

Very good reading... It gives a good and convincing explanation of why civilizations rise and why they are conquered.

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Another book that inspires some gaming awe in my is Stephen Dando-Collins' Caesar's Legion: The Tenth Legion. At the beginning of Caesar's career, he raised four legions from among the Roman settlers in Spain (the preferred recruitment area during the time of Caesar), the Seventh through the Tenth. The Tenth became his elite legion.

Battle after battle, through many a victory and the rare defeat, they followed Caesar from Spain through Gaul and Britain and even unto the shores of Egypt. Each battle winnowed out the unfit.

First to fall were the weak. Next, the careless. Next, the fatalistic. And finally, the unlucky. Only the best of the best survived to see the rise of Caesar completed.

If ever there was a description of a band of battle-weary adventurers, this is it.