I saw some threads on U-Boot books, and thought I should add a few I didn't see in the other lists. Some of these I came across by accident, and they are well worth exploring.
The U-Boats, from The Seafarers series of Time-Life Books - The reproduced artwork, many from Claus Bergen, are worth the price of the book alone, but there are many photographs as well. Some information is a little dated, but not a detriment. Of particular interest is the account by Leutnant Lothar von Arnauld de la Periere on the merchant ship he sunk, only to have it land on his submerged sub, and start dragging him into the depths. He was lucky to escape by blowing all ballast. Note to self - don't hang around too long under the merchant I just finished torpedoing.
Wolf, U-Boat Commanders in WWII, by Jordan Vause - This covers the conflict through the eyes and exploits of Juergen Oesten, Victor Oehrn, Karl-Fredrich Merten, Erich Topp, Wolfgan Luth, and several other luminaries in the U-Boat lexicon, and includes a chapter on the conflict between Merten and Lothar-Gunther Buchheim after the war, fighting each other on how the U-Boat men should be portrayed in history. Fascinating stuff and very well written.
The U-Boat Century, German Submarine Warfare 1906-2006, by Jak Mallmann Showell - This is a more technical book, with many photographs of subs and their interiors. Of particular interest, with Celeon in mind, is coverage of the modern German Submarine service.
Count Not the Dead, The Popular Image of the German Submarine by Michael L. Hadley - This one goes into great detail about the image of the German submarine commanders and their exploits, how they were received by their countrymen during the war, and how they were vilified after the war. There is quite a lot on the attempts by both factions, the pro and anti U-Boat personnas, to discredit the other's perspective. Really well written, but doesn't have a lot of wartime action. More cerebral, but well worth reading. Buchheim is prominent in the anti-U-Boat faction.
Sharks and Little Fish by Wolfgang Ott - This is one amazing book. I cannot recommend it highly enough. It was originally titled Haie und kleine Fische, German publication in 1956, and it is great! In its own respect as a wartime historical novel, it is better than Das Boot. It is one of the best written treatise of a German seaman, entering the service after working a fishing vessel, receiving training, serving on a mine sweeper as an officer midshipman, and ending up as an officer on a U-Boat. I was drawn deeper into this novel than I was for Das Boot, although I don't want to risk heresy and catch an errant lightning bolt for downplaying one of the best German submarine novels. I was stunned and amazed by Buchheim's work too. Suffice to say, this novel is excellent, and should be on everyone's personal shelf.
Hey, that's my take. Judge for yourself, but these books are top notch and make for fascinating reading.