Enigma: The Battle for the Code[0] is a very good read. It details the Polish achievements, the rush to save the Polish cryptographers out of France as the it was collapsing in 1940, and then goes into details with the many more details that gone into the cracking effort, besides Turing's team. Eg. tt shows the importance of weather report patterns in finding cribs that short-circuited the decryption (known clear text headers) and the importance of the submarine captured Enigma machines that showed the disk settings in the Navy setup (the Navy machine was much harder to crack than the Army ones). Not only a good read, but also very educating for anyone using cryptography today!
[0] http://www.amazon.com/Enigma-Battle-Code-Hugh-Sebag-Montefio...