This book, like many of the larger in the Osprey series, is an anthology of previously published Osprey titles. In this particular case, as another reviewer has noted, the publications include: Fortress 3: 'U-Boat Bases and Bunkers 1941-45', Warrior 36: 'Grey Wolf: U-Boat Crewman of World War II' and New Vanguards 51 and 55: 'Kriegsmarine U-Boat 1939-45 (1) and (2)'.
The sections on the development of the U-boats and their tactics between and during the war and quite rudimentary for anyone with more knowledge than that of a novice. Hence these would only be of value of someone new to the field. These chapters provide a very good and succinct overview albeit contain nothing new or anything said before. For those looking for more detail on tactics this reviewer would recommend another book in the Osprey series, "U-Boat Tactics in World War 2" as well as "The U-boat Commander's Handbook" (an English translation of an official Kriegsmarine handbook). For those seeking an overview of life on u-boats from the perspective of u-boat crews this reviewer recommends "U-BOAT Combat Missions" by Lawrence Patterson (one of the world's leading authors on u-boats).
The section on the U-Boat crews is a little more illuminating in that the original publication it was based on "Warrior 36: 'Grey Wolf: U-Boat Crewman of World War II", provided quite a bit of detail (especially photos and illustrations) regarding medals, dress, kit, etc. Hence for those interested in figurine building this would be a very good cost-effective resource. In addition, there is discussion on training, background of crews (especially how the Kriegmarine preferred to recruit those with mechanical skills) and how the caliber of personnel deteriorated over time as the war progressed (although no explicit discussion is given on how this affected U-Boat performance).
The section on fortified U-boat bunkers was probably the best. Very well illustrated. Really showed how these were constructed, how they looked when completed (complete with detailed cut-outs and photos), what functions they performed and how they survived allied attack (quite well, despite the enormous resources the allies spent on trying to destroy them via air raids). The only real weakness of this section of the book is that it puts forward that false impression that the bases were invincible when the fact was the allies, at least starting in mid-1944, were starting to make progress in their tactics. By using precision high-attitude bombing with giants bombs (in the 10 ton range and above) they were actually starting to penetrate these bunkers. Unfortunately, no discussion of this or its implications is even made (or of German counter measures that may have been able to render these tactics ineffective such as smoke screens).
In summary, the book provides a good succinct introduction to the beginner. For those with more knowledge, little would be gained through the 3 or 4 hours required to complete the books.