Here is my book list (so far):
Iron Coffins
Battle Beneath The Waves - Robert C. Stern
Operation Drumbeat - Michael Gannon
U-Boat War - Lothar-Gunthher Buchheim
U-Boats - Edwyn P. Hoyt
U-Boat Commander - Peter Cremer
Another Place another Time - Werner Hirschmann
Hunt and Kill (U-505) - Theodore P. Savas
and growing......