I'd recommend the following (both fiction and non-fiction), in no particular order:
Das Boot, Buchheim - Simply because it is a good read, and deservedly a classic.
Iron Coffins, Werner - Although it does contain the occasional bit of b*llsh*t, generally speaking it's rated by most people (with good reason) and it covers a broad spectrum, from new recruit, progressing through training, to lower ranking officer on a boat, right up to personal command of a U-Boat. And of course, it is already on your list!
Type VII U-Boats, Stern - Should be in everyone's library if they are interested in WW2 Atlantic U-Boats, as it contains a wealth of information on variants, tactics, weaponry, systems and just about everything else too, and has a great many cool pictures as well.
Donitz and the Wolfpacks, Edwards - This has a slightly misleading title, as it concentrates less on 'Donitz the man' than the title would seem to suggest. Nevertheless, as an overview of the U-Boat war with a great deal of emphasis on operations against convoys, it is a good choice, being that it covers in quite some detail many of the big actions by wolfpacks against the Atlantic convoys, including route maps, with sinking location marks on them and stuff like that. It also covers a lot about various orders issued to U-Boat men throughout the war by Donitz, including plenty of background detail on some of the more infamous ones, such as the order to attack 'rescue ships'. It's a bit of a hard read, but it does contain a wealth of info that rarely appears elsewhere.
Squadron/Signal publications Warships Number 1: 'U-Boats in Action' - Although aimed at modellers, this book is akin to a crash course in U-Boat development, covering all the major U-Boat types in considerable detail, with plenty of pictures and information on the major systems and variants.
Classic Warships Publishing, Warship Pictorial No 27: 'Kriegsmarine Type VII U-Boats' - Again primarily aimed at modellers, this is nevertheless a real gem. Anyone into U-Boats could hardly fail to be impressed with this book, as it is a comprehensive pictorial guide to the Type VII. It also features some really rare pictures which you simply will not find anywhere else (some of these in colour too), plus many very interesting technical diagrams which go a long way to improving ones understanding of where everything is on a Type VII U-Boat. Definitely one of the best books you can find on the subject.
Osprey Publishing, Warrior Series: 'Grey Wolf, U-Boat Crewman of WW2', Williamson/Pavlovic - If you want to know about stuff like uniforms, badges, rank insignia, personal escape equipment, ID cards and all that sort of thing, this is the one for you.
Osprey Publishing, Elite series: 'U-Boat Crews 1914-45', Williamson/Pavlovic - Sort of similar to the above title, but encompassing WW1 too, this is a brief but reasonably comprehensive overview of crews and equipment.
Anyway, there's a few to go at.
Chock