Enigma, by Robert Harris

This is a fictional story set in the universe of Bletchley Park, site of the Ultra project that lead to the breaking of several ciphers, including that of the Nazi's Enigma machine. The focal character of the story is a top-notch mathematician who had been instrumental in breaking the German Navy's Enigma-based "Shark" code, whose life is in shambles after a nervous breakdown brought on by an unsuccessful love affair. The story evolves around his re-recruitment at a point when suddenly "Shark" messages could not be cracked anymore and he's brought back to Bletchley Park. There he reaches out for his old love only to discover that she has mysteriously disappeared. The story is very tense, exciting, and somewhat didactic in that it takes place in a setting that has close ties with read history. It's a clever read that will leave amateur enthusiasts of cryptography scratching their heads and lovers of a good mystery flipping pages all the way to the end. Never mind the few historical inaccuracies, especially in what regards the fictional character Tom Jericho taking credit for Alan Turing's accomplishments: this is still a good read.