From the u-boat prespective you cannot go past Karl Doenitz's memoirs titles, "Memoirs Ten Years & Twenty Days". The ten years & twenty days refers to how long he was imprisoned for after the war.
Memoirs by - Dönitz, Karl Ten Years and Twenty Days
ISBN 0306807645
519 pages, 18 b&w photos
This book is one of the few books that will allow you to understand the German U-boat war as it was faught from the German side.
The Guardian I think best says it...
"The most distinguished of recent reminiscences on either the Allied or the German side. The author displays a powerful capacity of sustained concentration.
He writes with detachment and facility; he is both fair to his colleagues and his enemies, and possesses an effortless mastery of detail. He is, for a German officer, unusually honest in describing his relations with Hitler; and there is a refreshing lack of cant and fraud in his handling of the thorny problems of submarine warfare”
For me, this was one of those books that was impossible to put down.
I have heard great things of 'The War at Sea 1939-1945' by Captain S.W. Roskill DSC, RN. Now there are a few titles of simular name.
This is a 4 volume set!
I have this book, as I do the 1st and 2nd flotillas, for the last 6 months without having time to read them.