This is a fascinating and well written monograph about the struggle by allied codebreakers to solve the problem of the Nazi Enigma encryption machine. Shortly after the First World War, a German citizen developed a code machine but the German government had little interest in it so it was sold to commercial enterprises - in Germany and around the world. In 1928 the German Army became very interested in it and began using a modified version of the "Enigma" machine. In 1932 three Polish cryptanalysts began working on breaking the military Enigma codes and with some brilliant analysis were able to partially determine the architecture of the machines.
After World War II started, breaking the Enigma codes became a major goal of the British and American governments especially since German U-boats were wreaking havoc along the East coast of the United States and devastating American convoys carrying critical supplies across the Atlantic to Britain. Both began working on electro-mechanical machines ("bombes") to decipher the codes. The Enigma story gets more dramatic with codes from captured Nazi submarines and the intense work done by the WAVES operating the bombes in Washington, D.C.
Jennifer Wilcox has written an mesmerizing, clear, concise story that is as fascinating as any thriller. It has a great and understandable explanation of how Enigma encryption worked, and some good photos of the machines and the bombes used to break the codes.
This is one of the better books about Enigma.
Note: I got this book as a downloadable pdf from the NSA website in Fort Meade, Maryland.