This is the book.
This is the tooth. (I wouldn't want to swim with its owner!)
This is supposedly an authentic megalodon shark tooth. I say supposedly because I'm no expert, but Santa purchased it from a reputable seller. Little J has been reading, studying, and carrying this tooth around since Christmas. He is also very interested in other ancient animals and fossils, but for now this is his favorite.
Here is a link to a great study guide to megalodon. (midway down on the page you can download an educator's guide)