I second this recommendation, but also caution that it will only be of interest to someone truly interested in the details of the design evolution.
Some others that I have enjoyed
The U-Boat Wars 1916-1945 by John Terraine
This book goes into a lot of detail about the evolution of the U-boat tactics in the often overlooked inter-war period
U-Boat War by Lothar-Gunther Buchheim
This book is more focused on the crews and the personal experiences of the men. An interesting insight
The U-Boat Offensive 1914-1945 by V. V. Tarrant
This book has a lot of stats on the U-boats, the commanders, the career of the boat.
The afore mentioned book series by Clay Blair are always a good recommendation. However, Blair's books are not what I consider an "easy read".
Good luck with this. Collecting U-boat books is a terrible and expensive hobby.
