Probably the best sub book on WW2 boats is Erminio Bagnasco's book Submarines of WW2. It's available from Borders and B&N. It covers all the classes from all the combatants, obviously paying alot of attention to German, Japan, the US, Great Britain, and Italy.
It's relatively inexpensive, in the $25-30 range.
Jeff Herne
Modelwarships.com