This book is really worth getting if you can find it somewhere for a fair price. It's almost worth it for the appendix alone where Marian Rejewski himself describes how they broke the original Enigma code.

http://www.amazon.com/Enigma-German-Machine-Foreign-intellig...

A really great book [1] which details how Marian Rejewski, Jerzy Rozycki and Henryk Zygalski exploited a lot of these human errors and managed to keep up with the changes of the Enigma machine.

[1] If you can find it somewhere for a fair price, http://www.amazon.com/Enigma-German-Machine-Foreign-intellig...