Since we're on the topic of great U-boat books, I HIGHLY recommend "Iron Coffins" by Herbert Werner. He writes about his first hand experiences serving on five different U-boats from 1941-1945. It is well written and hard to put down; exciting, intense and somber all at the same time.
During one of his tours in '43, the author describes how their sub was submerged and under attack by several destroyers for almost 35 hours...where they eventually reached a depth of 280 meters! The book also demonstrates how often and threatening air attacks were in the last few years of the war.
Published by Da Capo Press. More info here...
http://www.perseusbooksgroup.com/dac...sbn=030681160X