Required:
Ewen Montagu, The Man Who Never Was, entire.

Suggested:
David J. Alvarez, Secret Messages: Codebreaking and American Diplomacy, 1930-1945 (Lawrence: University Press of Kansas, 2000).

Stephen Budiansky, Battle of Wits: The Complete Story of Codebreaking in World War II (New York: Free Press, 2000).

David Kahn, Hitler's Spies: German Military Intelligence in World War II (New York: Macmillan, 1978).

David Kahn, Seizing the Enigma: The Race to Break the German U-Boat Codes, 1939-1943 (Boston: Houghton Mifflin Co., 1991).