Navy Department Library Reading Room (World War II)
Navy Department Library Reading Room (World War II) – https://web.archive.org/web/20141029213005/http://www.history.navy.mil/library/online/readingroom.htm#worldtwo
Representative questions that can be answered with this resource:
- What could an American POW be expected to be asked about by his Japanese captors?
- What was a breasting float and what was its function?
- How did the Germans react to the loss of the Bismarck?
- What was involved in crossing the Rhine in 1945?
A lengthy list of alphabetically sorted links to reports, manuals, guides, after action reports and more. Just eyeballing the list can be daunting but worth it. I have pulled out about half of the resources and sorted them into topics that I think will be of interest to the writer:
First hand accounts
- Recollections of Ensign Leonard W. Tate Recounting His Service in the US Navy Including the Invasion of Southern France and with SACO [Sino-American Cooperative Association] in China During World War II
- Recollections of Lieutenant Commander William Leide Concerning the Crossing of the Rhine Rver in 1945.
- Recollections of Lieutenant Wilton Wenker and Lieutenant Elby Concerning the Crossing of the Rhine River in 1945.
- Recollections of USS Pampanito’s rescue of prison ship survivors by Lieutenant Commander Landon Davis 
- Recollections of Vice Admiral Alan G. Kirk Concerning the Crossing of the Rhine River in 1945.
- Conduct of the War At Sea  by Admiral Karl Doenitz
- G.A.F. [German Air Force, Luftaffe] and the Invasion of Normandy 1944 by Oberst Walter Gaul
- German Commanders Wilhelm Keitel and Alfred Jodl on the Invasion of Normandy in 1944.
- Interrogation of General Alfred Jodl Concerning the Invasion of Normandy in 1944.
- Japanese Story of the Battle of Midway
- Sinking of the German Battleship Bismarck as described in the B.d.U. [Commander U-boats] War Log, 24-31 May 1941
- U-505, Documents Captured on German Submarine
- World War II Invasion of Normandy (1944) – Lieutenant General Rudolf Schmetzer – Report Regarding the Construction of the Atlantic Wall
- Japanese Interrogation of Prisoners of War
Know Your Enemy
- Japanese Naval Ground Forces “Know Your Enemy!” [CinCPac-CinCPOA Bulletin 11-45, 1945]
- Japanese Radio Communications and Radio Intelligence: “Know Your Enemy!” [CinCPac-CinCPOA Bulletin 5-45, 1945]
- Pocket Guide to Hawaii 
- Pocket Guide to Japan 
- Pocket Guide to Netherlands East Indies 
- Pocket Guide to New Guinea and the Solomons 
- Short Guide to Iraq
- Anti-Suicide Action Summary [COMINCH P-0011]
- Battle of Guadalcanal 11-15 November 1942 Combat Narrative – Office of Naval Intelligence
- CIC [Combat Information Center] Operations on a Night Carrier
- General Instructions for Commanding Officers of Naval Armed Guards on Merchant Ships 1944
- Handbook of First Aid Treatment for Survivors of Disasters at Sea 
- Shiploading: A Picture-Dictionary of Shiploading Terms
Again, this is only about half the resources on the page, if that. If you are writing a war story, either set in World War II or a fictional war, time with this page is well spent.