The books about naval warefare that i would recoomend and about strategy of war at sea. On the top of list:
"The War at Sea" by captain S.W. Roskill in 4 volums ( it 5 books as vol 3 is in 2 parts )
Those who had "10 years & 20 days" by K.Donitz knows that author often apply to Roskill's book.
Then ofcours for those sub - maniacs and U-boat war, there is so many titles to recommend. For me there are 3 fundamental
"Hitlers U-boat War" by C.Blair ( 2 volums)
"U-boat operations of the second world war" by K.Wynn (2 volums)
"The U-boat war in the Atlantic 1939 - 45 " by. G.Hassler
There is one more author that i do support on his books: R.Woodman and his "Arctic Convoys 1941 - 45" & "Malta Convoys 1940 - 43"
These are my fundaments of sources which i would recommend to everyone. Additional those guys from London can brows Leicester Square area with so many book shops which offer several different titles. Last time i was there could not pass indifferent. I effect i did comeback home with about 10 books, lost fortune and good relationships at home for few days. However complete history of 1st and 2 nd U-boat flotillas by Lawrence Patterson are at my home, and many other books which i will not describe here. If any of You want to buy a really good source of inforamtion look for Conway publications.