For many years the Marine Biological Laboratory at Woods Hole (on Cape Cod,
in Massachusetts) offered an extremely useful publication called Key to
Marine Invertebrates of the Woods Hole Region.  As you can tell from the
title, it covers more than mollusks.  I just went to their website and was
disappointed to see that they have removed the book from their publications
list.  The publication is certainly dated, but I still find it very useful.
The mollusks that were in the area when it was published are still there
now.  And this is not an area like the Philippines where a hundred new
species a year are being described.  Perhaps if you contact them through
their website, you can obtain a copy.

http://www.mbl.edu

Regards,
Paul M.