Mary,

It depends on your specific interests and willingness to spend. The Bahamas
book is strong on micro shells as well as other less obvious mollusks. The
price will be very steep for most casual to moderate collectors.

I really like the Audubon book for the quality of the photos (although not
for the way it's organized). There's an Abbott paperback on North American
seashells that came out in 1968 with a light green cover and I got a lot of
mileage out of that. The large-format paperback on Florida Seashells looks
very good for beginners, if the Southeast US is your region of interest.
Caribbean Seashells by Warmke and Abbott is very worthwhile even if dated.

Scour those used bookstores because you never know when you'll come across
something.

Good luck!
David
Durham, NC