Another good one would be "Seizing the Enigma: The race to break the German U-boat codes" by David Kahn. It's an excellent book on that little slice of the Battle of the Atlantic.
Other than the previously mentioned "Iron Coffins", another good book written by an actual U-boat commander is "U-Boat Commander" by Peter Cremer. I don't think it's still in print, but you should be able to find a used copy somewhere.
Of course, for the comprehensive overview of the whole U-boat campaign, there is nothing that compares to "Hitler's U-boat War", volumes 1 and 2, by Clay Blair. If you can only get two books, get those two.
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The U-Boat Commander of Love
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