If you're looking for an overview of cryptography in an easy-to-digest format, take a look at:

  The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography

Amazon link: http://www.amazon.com/Code-Book-Science-Secrecy-Cryptography...

It is light on the math but Simon Singh does a great job reviewing cryptography over the last 4000 years. It was a fun summer read.