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'The Elegant Universe' by Brian Greene.

It's a book about physics, dealing with the theories of special relativity and quantum mechanics and the problem of trying to fit them under a single roof. It focuses on the idea of 'string theory' to do this.

It's not a text book, it's popular press (it's actually a pretty easy read) and almost every page has some fact that nearly bowls me over. He describes everything very conceptually using analogies that are easily digestible, so it's understandable if you're no good at math (like me). Definitely a must read if you're into sciency type stuff.