The Universal Computer. It's a remarkably readable walkthrough through the lives and ideas of mathematicians whose work indirectly led to the Turing machine: Leibniz, Boole, Russell, Cantor, Godel, and a few others. If anyone else can recommend more books on the mathematics that led to computing I'd love to hear about them.
http://www.amazon.com/The-Universal-Computer-Leibniz-Turing/...