When in doubt, you can never go wrong buying a book by Jak Mallmann Showell whom I consider the foremost authority on U-Boats in the world. Although he is English, he was born in Germany in 1943 and his natural father was killed in action that same year aboard a Uboat. This is an excellent book on many levels but first and foremost it reminds us that while the Germans did not have a code-breaking success anywhere close to Ultra, they were damned good code breakers and broke almost all Allied maritime ciphers except for the very top ones. ( )