I just started reading
Business in Great Waters: U-Boat Wars, 1916-45 by John Terraine.
I am only about 100 pages into it. But so far it is a very interesting history of WWI submarine warfare. It goes into nice, cited, detail on the difficulties that Germany had concerning the decision for unrestricted submarine warfare. I did not know there was that much dissent in Germany about this.
It also goes into detail not found in other books on how the decision for unrestricted warfare came about and what the British did to escalate it.
I am only about 1/5th through the book. But if it continues as it started, I am glad I added it to my library.
Good index and nice citations. I like that in a history book.