Reddit reviews U-Boat War

If you haven't read the book, I'd highly recommend it. Also, the book's author, Lothar-Günther Buchheim, was a journalist / artist who served on a u-boat and his photographs were collected in a very interesting book.

U-boat.net for a very good online source on German submarines in both World Wars, more encyclopedic than a book, however.

Wolfpack and Battle of the Atlantic are two very readable and good books for a general overview, but there are certainly more academic or exhaustive books as well.

The U-boat War is an oldie but a goodie, written by the same person who wrote das Boot, the fictional counterpart.

On the American side, several submariners wrote very good memoirs. Thunder Below by Eugene Fluckey gives a great overview of both life at sea in an American submarine in the Pacific, but also of operational details, how the patrols worked, etc.