The U-Bootwaffe of the Kriegsmarine had the largest submarine force at sea of any nation in history, before or since.  Naturally, everyone knows of the Type VII and its variants as well as the Type IX and its variants.  This book reveals the rare types with some futuristic experimental boats.   https://www.sharkhunters.com/books/ will tell you all about this book and you will learn more about this fascinating history, available nowhere else.  For information about Sharkhunters, the official history of the U-Boats, go to https://sharkhunters.com/