Dr. Thomas R. Johnson's American Cryptology During the Cold War, Books I, II and III have been recently released on the National Security Archive page. While apparently quite a bit has been redacted by the NSA ("No Such Agency", to some), this is nevertheless an interesting treatment on the NSA's SIGINT signals intelligence and COMINT communications intelligence branches during perhaps the hottest period of the Cold War, the Cuban Missile Crisis.

Books I, II, and III can be downloaded from the National Security Archive in PDF form.