Seizing the enigma - David Kahn

For almost four desperate years, from 1939 to the middle of 1943, the British and, in time, American navies fought a savage, losing battle against German U-boat wolf-packs. The Allies might never have turned the tide without an intelligence coup. The race to break the German U-boat codes is one of the greatest untold stories of the Second World War. Kahn expertly brings this tale to life in this updated edition of his classic book.
Soon after war broke out, Hitler's U-boats began to sever Allied lifelines. In the grey wasteland of the North Atlantic, submarines prowled; at night, the sky lit up with the flames of torpedoed and sinking merchant ships and tankers.
To meet the growing crisis, ingenious amateurs joined the nucleus of dedicated professionals at Bletchley Park. As the Battle of the Atlantic raged, they raced to unlock the continually changing German naval codes. Their mission: to read the U-boat messages of Hitler's cipher device, the Enigma.
Critical to their success was a series of raids at sea. U-110, captured intact in the mid-Atlantic, yielded an example of the Enigma machine itself - as well as a trove of secret documents. The weather ship Lauenburg, seized near the Arctic ice pack, provided code-settings for an entire month. In the Mediterranean, two sailors rescued a German weather cipher than enabled the team at Bletchley to solve the Enigma after a year-long blackout.