Those who are interested in the history of SIGINT and old cipher machines should visit the NSA's Historical Publications. There are publications on WW2 SIGINT history, mathematics and cryptology, cipher machines and breaking Enigma. There are also Cold War papers about Aerial Reconnaissance, the Cuban Missile Crisis and the notorious VENONA project to break Soviet cipher traffic. In the Miscellaneous section you will find some papers on cryptology and the NSA's history. Those who are not familiar with the NSA and what these spooks are doing can visit the NSA website or the Wikipedia NSA article and learn some more about the guys in the Puzzle Palace.