I just finished
Operation Drumbeat by Michael Gannon, which I thought was excellent, and
U-Boats under the Swastika by Jak P. Mallmann Showell, which is a good reference book.
I'm starting my re-read of
U-Boat Ace: The story of Wolfgang Luth by Jordan Vause.
I don't know what it is, but something has me interested in reading about U-boats...