Leon,
Surf over to this link:
http://pigboats.com/dave3.html
These are the books that I recommend. The submarine design books can get pretty technical, so you may have to skim them until you get more knowledgeable.
This list is not all inclusive. There are books out there that I did not list that are good to read. My area of specialty is the U.S. boats, unfortunately I can't help you with the German side.
Dave