Links last checked April 2021. New sources are popping up frequently.
Documents related to the Intelligence War in the European Theatre of Operations at Hyperwar. Scroll down the page.
Many declassified documents at NSA Cryptologic Histories
Documents at the Internet Archive. Many search terms possible including Enigma and Bombe
Extracts from COMMENDATIONS Received By U. S. Naval Communication Intelligence Organization Since 1 January 1942
An index to the large collection at the Naval History and Heritage Command.