I presently have a collection of over 80 books on U-boats, but I have only read about one-half of them so far. I try to buy hardcover first editions if possible. But some of my earlier purchases were paperbacks.
While I collect most all U-boat books, I prefer reading books that are personal memoirs or first hand accounts of u-boat warfare.
A couple of my favorite books have been:
Iron Coffins: A Personal Account of the German U-boat Battles of WWII by Herbert Werner
U-boat Commander: A Periscope View of the Battle of the Atlantic by Peter Cremer
Operation Drumbeat: Germany's U-boat Attacks Along the American Coast in WWII by Michael Gannon
The Laughing Cow: A U-boat Captains Story by Jost Metzler
Steal Boat, Iron Hearts: A U-boat Crewmans Life Aboard U-505 by Hans Jacob Goebeler