In my current assignment I work a lot with security issues. Recently I've had some work involving cryptography. As I have no experience with this from the past it was kinda hard getting started, but this book really gave me a good introduction on the subject.
It walks you through the basics of the history, block ciphers, stream ciphers and public key cryptography. It also has some chapter about the problem with public keys and Public Key Infrastructure. On some of the parts it goes a little deeper but mostly its a on the surface of things. It gives really good real world examples and is in a small format so you can bring it on the buss or the train.
Bottom line. Great book. Cheap book. Buy it if you want to get an intro to cryptography.