You could do worse than to get Bucheim's 'other' book:
U-Boat War; though it's pretty rare these days.
The books you want to start withe are Clay Blair's two books:
http://www.amazon.com/s/ref=nb_ss_gw...rds=clay+blair
I would recommend Peter Padfield's
War Beneath the Sea for an overview of the war from all sides, also getting rare. Get it used through Amazon for $15.00.
Then you can start here:
http://www.amazon.com/s/ref=nb_ss_gw...rds=u-boat+war