Enigma by Robert Harris

What the cover says:
March 1943, the war hangs in the balance, and at Bletchley Park Tom Jericho, a brilliant young codebreaker, is facing a double nightmare. The Germans have unaccountably changed their U-boat Enigma code, threatening a massive Allied defeat. And as suspicion grows that there may be a spy inside Bletchley, Jericho's girlfriend, the beautiful and mysterious Claire Romilly, suddenly disppears.

I picked this up because of my interest in codes and more recently Bletchley Park (particularly as it's not that far away) and because I couldn't be bothered to stay up to watch the film when it was on a few weeks ago. I'm very glad I didn't though as I'm fairly confident that the book is way better than the adaptation.
It's a classic wartime thriller with spies and codes and a tormented genius for a hero. Although it starts off fairly gently, the tension builds and builds to a breathless finale and once I got past halfway through I couldn't put it down.

9/10