My Favorite guides for shells of that region are: "The Peterson Field
Guide to Shells of the East Coast, Gulf Coast and the West Indies"
Collectable Florida Seashells by Abbott ( a simple but good one)  and
Florida's Fabulous Seashells by Pete Carmichael. The latter one  has fantastic
pictures of shells and other invertabrates ( and few fish) in it.  The one I
use the most  is the peterson guide.
 
My favorite place to shell in that region is the state park on North Captiva
Island. This island is inhabited on the north side.  Another good place around
Pine Island Sound  is Caya Costa which is uninhabited and rarely visited.
Many shelling trips take you there or North Captiva.
 
On Sanibel the  best place  that I know of ( I can't believe I am sharing this
info) is
on the Bowman Beach stretch about a mile past the Snook motel. Its smack in
the middle of the island and everytime I have gone there  I have come back
with  incredible shells.  The Beach is pretty narrow in there ( at least it is
in December I don't know what the beach migration patterns for the rest of the
year)  Pretty much anywhere in the middle of the island is a great shelling
place. You just have to walk and search. You can walk down there from the
Bowman public beach but its about 2 miles. Some rare shells that can be found
on Sanibel is the Famous Junonia ( Scaphella junonia) other Scaphella volutes,
Lion paws (Lyropecten nodosus )  and Golden Olives ( Oliva citrine)   Last
year I found one half to the Lyropecten  in the masses of rubble on the
stretch of Bowman. The best time is at the crack of dawn, low tide after a
good storm from the northwest ( the usual shelling conditions)
Another good place is Blind pass on the Captiva side. That is where the
islands seperate. There are tons and tons of huge piles of shells, most are in
good condition you just have to sift through it. My favorite time to go there
is sunset. You could probably dock just off shore where the water is about 3
ft deep.  By the way  you will find wearing  shoes on the beach on Sanibel
very helpful
 I hope this helps you out. I wish I were going, but my plane ticket is on
Christmas Day. Have a blast.
 
Sarah
 
http://www.geocities.com/SouthBeach/Tidepool/8845