The Book of Shells is a book that Squidward owns. Squidward reads it. It is just seen in the episode "I ♥ Dancing."
The book is dark green with yellow letters on it that say Book of Shells. The inside of the book is never seen, but it can possibly be about shells for snails.
Role in the episode
When SpongeBob is supposed to Moonwalk on Squidward's treadmill, Squidward goes away to read this book. It is unknown what it is about, but it can be assumed that there might be snail shells in it. It may also be a guide book full of seashells. He might have been reading because he wanted a shell for Snellie. Squidward obviously had this in his book room.