I focus primarily on FL and Bahamian shells.  To help me with these
I find I need a combination of books:

Peterson Field Guide / Shells of the Atlantic and Gulf Coast and the West
Indies ( Abbott and Morris)

The Audubon Society Field Guide to N. American Seashells (Rehder)

Golden Field Guide ( Abbott ) Seashells of N. America  (this must
be the "little green Abbott" people have mentioned. Mine was published
1968. Great illustrations - these do better than black and white photos).

Sea Shells of the W. Indies ( Michael Humfrey)

Caribbean Seashells ( Warmke and Abbott)

American Seashells 2nd ed ) (Abbott)

( I usually find I have to combine books in order to cover what I find
on any particular trip).

I also like Shelling San Sal by Betty Lawson,  New Caribbean Molluscan
Faunas by Petuch, Florida's Fabulous Seashells, Clean Your Shells and
Other Sea Life by Betty and Robert Lipe, Collectible Fl Shells by Abbott,
among many others.

Karen