The Code Book by Simon Singh

For me this is the definitive work, though Mr. Singh goes to some length to declare that his is not the definitive work on the subject of Cryptography. (I love a guy who 's humble.) I first read this book about a year after it came out and of course that would be a year after the infamous 10 stage challenge began.  (I still managed to solve the first six stages a few days before the 4 Swedes with the Supercomputer took the whole thing.)  I had been a casual fan of cryptography and a cryptogram lover for many years, but really didn't have a good basic knowledge of the history of the discipline.  This book will give you that. In the process I became obsessed. For weeks I couldn't think of anything but solving the next stage. If you're curious about cryptography, but not sure if it warrants more of your time and valuable brain space, then do not read this book; I found I couldn't turn away, unless it was to read other sources on the subjects he raised. Reading this book could easily leave you with feelings of profound respect for its author, but could just as easily leave  you feeling just cocky enough to want to play in the Big Game.

The Code Book for Young People
How to Make it, Break it, Hack it, Crack it

Also by Simon Singh, I haven't read this one yet, but soon will. I'll let you know if I learn how to Hack it or Crack it!