"The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography" http://www.amazon.com/The-Code-Book-Science-Cryptography/dp/... has a chapter about the Enigma. This book is easy to read as a nice intro to many crypto topics.

"Alan Turing: The Enigma" http://www.amazon.com/Alan-Turing-Enigma-Andrew-Hodges/dp/08... , supposedly the best biography of Turing, I enjoyed it. There's quite a bit on his work on the Enigma but not technical.

I was going to recommend the Hodges biography of Alan Turing, but it appears to be (sadly) out of print.

http://www.amazon.com/Alan-Turing-Enigma-Andrew-Hodges/dp/08...

[edit: In the US. Someone else posted the link to the UK edition, thanks!]