Want more money? Write a book!

Have you ever noticed that when a movie becomes successful, a line of books come shortly after. Disney did this with Pirates of the Caribbean. They created a series of young adult books about Jack Sparrow. I remember reading book versions of The Mummy Returns and Hallmark's Arabian Nights. And today I came across a new series of books based on the hit TV show Glee. You can visit the site for the book, which FOX advertised on Facebook, here.

How successful are these knock-off books? What have you done similar to this?