Some time ago I found a website where you can buy US-Navy manuals from WWII in pdf format.
http://www.history-on-cdrom.com/home.htm
Theres also a free version of the Fleetsub manual from 1946.
http://www.maritime.org/fleetsub/
Looks like the real literature for a UBoat skipper to skim through while on patrol. I am tempted but it isn't cheap.
Has somebody ordered their stuff and can
recommend it?