Some of my favorite books
Tideland Treasure, by Todd Ballantine: I have the old black and white one, but this is an incredible resource for learning all about the creatures that call the beach home.
Nature Guide to the Carolina Coast is a great overview of the common birds, crabs, shells, etc.
Seashore Animals of the Southeast is a more scientific, in depth look at the biology of your beach finds.
SCDNR published a great resource years ago that is available in a download here.