I love Simon's Singh's books. I started with Fermat's Enigma, although I'm pretty sure the version I read was called Fermat's Last Thereom. Then, I read the Code Book, followed last year by Big Bang.

The Code Book is about the history of codes and ciphers, but mostly ciphers. He starts with some of the oldest, most basic ciphers and moves through history to the present (okay, 1999, but close enough). Along the way, we meet characters like Mary Queen of Scots, Alan Turing, and the Navajo code talkers. My favorite parts were the breaking of the German Enigma, the decipherment of Linear B, and the lengths the British government went to so that the Germans wouldn't know their ciphers had been broken. It's all engaging, even the part about the future of cryptography that I found boring the first time I read it. If you like secrets, math, and history, you'll love it.

Here's the Amazon link: