For the U-Boat side of things...
U-Boats at War by Harald Busch. Out of print, but not hard to find. Contains a concise history of the U-Boat war, mixed with detailed stories of many notable patrols (Prein's Scapa raid, a Q-ship encounter, Erich Topp's patrol in U57 - a type II, etc.) Also has Wolfgang Luth's lecture
Command of Men In a U-Boat. An amazing book.
Sharks and Little Fish . Since you're reading
Das Boot, you might like the contrast that this book provides. It's also fiction, but it presents a far less romantic view of U-Boat life and operations.
During the war Buchheim was a professional propagandist whereas Ott was a submariner - a fact which I think accounts best for the difference in tone of these two books.
and this...
The Submarine: A History by Thomas Parrish. Title says it all. A very readable history of the submersible warship and the effect it had on naval warfare