The 39 Clues is a series of adventure novels written by a collaboration of authors, including Rick Riordan, Gordon Korman, Peter Lerangis, Jude Watson, Patrick Carman, Linda Sue Park, Margaret Peterson Haddix, Roland Smith, David Baldacci, Jeff Hirsch, Natalie Standiford, C. Alexander London, and Jenny Goebel. It consists of four series, The Clue Hunt, Cahills vs. Vespers, Unstoppable, and Doublecross. They chronicle the adventures of two siblings, Amy and Dan Cahill, who discover that their family, the Cahills, has been the most influential family in history. The first story arc concerns Dan and Amy's quest to find the 39 Clues, which are ingredients to a serum that can create the most powerful person on Earth. This series' primary audience is age 8–12. Since the release of the first novel, The Maze of Bones, on September 9, 2008, the books have gained popularity, positive reception, and commercial success. , the book series has about 8.5 million copies in print and has been translated into 24 languages. The publisher of the books is Scholastic Press in the United States. Steven Spielberg acquired film rights to the series in June 2008, and a film based on the books will be released in 2016. The series also originated tie-in merchandise, including collectible cards and an interactive Internet game.