Rick Riordan – yet another superstar in the young adult market – also made $21 million this past year. Riordan writes the bestselling "Percy Jackson and the Olympians" series, recently made into a film in the US. The series stars Percy Jackson, demi-god son of Poseidon, who finds out that the Greek/Roman gods (and monsters) are still alive and well in the modern world, and he must work to both discover his heritage and protect the world from the forces of evil.