The German Enigma Cipher Machine
AuthorB. Winkel, C. Deavours, D. Kahn, D. Kruh
This focused collection of essays and documents about the Enigma machine and wartime cryptography in general appears to target a more expert readership than Simon Singh's excellent work covering similar ground, The Code Book.
The German Enigma Machine includes detailed accounts of the construction and management of installations such as Bletchley Park, and their roles in the battle for secure comms and code breaking, as well as more general pieces on the role of encryption in warfare, and what formed the groundwork for much of modern best practice in cryptography and security.
The sections on why Enigma and others failed are perhaps the most compelling: concerted code-breaking, grand misjudgements and luck were all factors, and remain relevant today.
Although dense at times, this is highly recommended for anyone with a particular interest in cryptography, or even WWII.