"Hitler's U-Boat War: The Hunters, 1939-1942" by Clay Blair. I picked it up for $5 in new condition at a Tampa Bay used bookstore and have been grinding through its 864 pages at a steady clip. http://www.amazon.com/Hitlers-U-Boat.../dp/0679640320

This is the first volume of a massive two-book effort. The second volume details the period of 1942-1945 when fortunes were reversed and the German vessels became the hunted.

Like John Lundstrom's "The First Team: Pacific Air Combat from Perl Harbor to Midway", Blair's work is very detailed and draws from a wide range of sources to paint a picture that contrasts those put forth by many historians.

Blair asserts that the German WWII U-boat effort was not as effective as many historians otherwise believe, and he does so with a really good narrative style which has made the book much easier to read than Lundstrom's resource.

I always found submarine warfare an interesting, if only peripheral topic to read on, but I have really enjoyed the various movies like Das Boot. Blair's work is a good book that covers a lot of ground while still providing details where they matter--at least for my taste.