All good suggestions... I've read quite a few, but picked up quite a few suggestions as well!
I will say one thing...
Avoid this one:
Submarine Conflict 1939-1945 - War Beneath the Sea
(Peter Padfield)
It is a very promising looking book, but full of foolishness.... and I've only made it to chapter 3 so far.
It opens with a description of the sinking of the S.S. Athenia.
While most reports attest that the Athenia was sailing as a darkened ship and zig zagging in the twilight hours, Padfield asserts that the u-boats periscope "broke the surface into the sunlight"... and Lemp then ordered the attack.
He goes on to further claim that after the initial torpedo shots, Lemp then surfaced and attacked the ship with his deck gun in an attempt to "target the radio room".
Statements in the first chapter about U-boat crews differ completely from the statistics in Timothy Mulligan's excellently researched and referenced "Neither Sharks, Nor Wolves".... and Padfield makes several claims about Donitz's plans and dispositions that are the opposite of Donitz's own remarks in his memoirs - which I had just finished reading previously.
The straw that broke the camel's back was a reference to the use of plastics in 1935 on Italian 'human torpedoes' by a pair of engineers who "scrounged parts from the submarine dockyard" in Italy.
News to me.... who would have thought plastic would be so widely available in 1935?
Long story short: It comes in a nice package, but so far totally unimpressive in every regard.
Save your money!