My favorites (I have A LOT of u-boat books)
1. The U-Boat by Eberhard Rossler (technical history).
2. U-Boat War by Buchheim (the actual photos from his patrols)
3. Hitler's U-boats by Clay Blair (Both volumes) General history
4. Wolf by Jordan Vause (I love his chapter about Buchheim)
5. Torpedo Juction by Hickman
6. U-boat commander, by P Cremer or Track of the Grey Wolf by (I forget) U-124 Mohr/Schultz
This just scratches the surface of my submarine library
