The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography
This is a fascinating and mind-bending read. Simon Singh does an excellent job of taking a complex subject and making it into simple portions. It tells you what codes are, and how they are broken. It's quite a history lesson too - codes and ciphers have been in use since Caesar, and Egypt as well. You'll learn progressively harder (yet fascinating) code methods as well as cracking methods. It's very well balanced between learning and just fascinating information - never too hard to understand. There have been a few developments since this was written, but only a few. I recommend it to anyone who wants to keep their mind active or simply learn about an historical subject.
Great for a geek