[mookiemookie]

It depends on if you're looking for U-boats or U.S. subs in the Pacific

As far as U-boats go, Iron Coffins is a great read. Also good is Operation Drumbeat by Michael Gannon about the time when Germany sent U-boats to the east coast of the U.S.

Blair's Silent Victory is awesome for a blow by blow account of the U.S. sub war, as are both volumes of his Hitler's U-Boat War for the German side, but they are not written in narrative style and while comprehensive, can be a bit dry in parts.

I'd go with Clear the Bridge, or even The Bravest Man by William Tuohy. Both are about Dick O'Kane, one of the great U.S. skippers of WW2. Great reads, both of them.