The two NSA pamphlets I mentioned previously, “Solving the Enigma – History of the Cryptanalytic Bombe” and “The Cryptographic Mathematics of Enigma” are available from the NSA website as free downloads, along with quite a few other publications related to WWII cryptography. There are also publications available on cryptography in other eras: Korea, Vietnam, Cold War, etc.
I personally like having the printed versions to have and to hold (and you can request them by email), but this is a gold mine for the impatient person who really wants to know the “History of the Cryptographic Branch of the People’s Army of Vietnam 1945-1975“.