The 39 Clues is a series of adventure books, combining reading, online gaming, and card collecting. Published by Scholastic, the main part of the series consists of ten books about the adventures of siblings Amy and Dan Cahill. They agree to the challenge in their grandmother Grace Cahill's will and search for 39 clues scattered around the world. Rick Riordan wrote the first book and the main story arc, however, other authors wrote the remainder of the books in the series. As of May 25, 2010, nine have been written by Rick Riordan, Gordon Korman, Peter Lerangis, Jude Watson, Patrick Carman, Jude Watson (again), Peter Lerangis (again), Gordon Korman (again), and Linda Sue Park. The final book will be written by Margaret Peterson Haddix