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 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. Squidward keep the book in his book room.