I don't want to make repeat posts on a subject but didn't see this listed as I searched threads and thought some might find it interesting reading.
https://maritime.org/doc/uboat/index.htm
It's the "updated" 1943 version of the German U-boat Commanders handbook that discusses tactics at that point in time but allies already were aware and countering them.
Interesting view on how the U-boats were to be used and not used. Would like to read previous versions (1939-1942) to see what differences existed.
This is a link from the
USS Pampanito site, which is moored in San Francisco and open to tours. Site has a lot of great links--check them out!