[Donner]

Here's a few more suggestions...

Another Place, Another Time: A U-boat Officer's Wartime Album by Werner Hirschmann. Excellent memoir with many personal photos of his wartime experiences.

Shooting the War: Memoirs of a World War II U-boat Officer by Otto Giese. Great narrative of a man who starts as a junior officer aboard the ocean liner SS Columbus and finishes as a POW.

Steel Boats, Iron Hearts: The Wartime Saga of Hans Goebeler and the U-505 by Hans Goebeler. Interesting read.

Hirschfeld: The Story of a U-boat NCO 1940-1946 by Wolfgang Hirschfeld and Geoffrey Brooks. Great memoir written from Hirschfeld's secret diaries written aboard U-109 and U-234.

U-boat Commander: A Periscope View of the Battle of the Atlantic by Peter Cremer.

The Odyssey of a U-boat Commander: Recollections of Erich Topp by Erich Topp. An interesting book written by one of the most famous of U-boat aces. Only about half of the book is devoted to his wartime experiences, but it is fascinating nonetheless.