This book is a very good review of the history on encryption and explains the basic principles involved. It is a lot like David Kahn's The Code Breakers, but is available for a good deal less. Beginning with Herodotus and some secrecy measures from The Persian Wars, it then moves forward with Arab scholars, medieval developments, and right up to asymmetric public key encryption used today. Highly recommended for anyone who wants to get an overview of what the issues are, but is not looking to dive into the mathematics.