Enigma by Robert Harris

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 disappears.