How did Ella's book change for the elves?

The elf, Slannen, commented on how tiny the elves were in the story The Elves and the Shoemaker. He was surprised they could fit into a shoe. He turned a few pages, and when he went to turn back to the story about the shoemaker, the story had changed. It had turned into a story about a walrus and a camel. Slannen then realized the book was fairy-made.