Thats an easy one.
''Business In Great Waters: The U-Boat Wars 1916-1945''. John Terraine, Pen & Sword Books, South Yorkshire, UK , 1989.
As a bit of a plug for Pen & Sword, I discovered some years ago that this was a rather decent literary clearinghouse for no end of military history from The RAF in North Africa, to the Somme, Midway and Trafalgar, and even accepts submissions from writers in this genre.
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