A few suggestions:
Guns, Germs and Steel by Jared Diamond. You may have read and/or heard of this one anyway, but it's still a really neat book - even if I don't agree with all of his ideas myself. It tries to answer the fundamental question: why did some civilizations(i.e., European) enjoy such technological superiority over the other cultures and societies they encountered(and conquered)? Or, to put it a little differently, why did Charles the Fifth's men take the Inca Atahualpa prisoner instead of the other way around?
Another neat one along the same lines is Plagues and Peoples by William Hardy McNeil. I just read this one myself recently. Basically the author argued(it was written in the 1970s) that the impact of epidemic disease on history had been greatly underestimated and might explain, among other things, the fall of the Roman Empire.
If you're a nerd like me and you enjoy reading textbooks(admittedly, not everyone does), consider
this book on cell biology - nice introductory book that covers the whole deal. Neat stuff if you are so minded.