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 trying to thwart the other Cahills to get the clues, which will make them "the most powerful and influential person(s) on the planet". Amy and Dan agree to the challenge in their grandmother Grace Cahill's will and search for 39 Clues scattered around the world. Continuing books will be about the rising of a new family, the Vespers. Rick Riordan wrote the first book and the main story arc; however, other authors wrote the remainder of the books in the series. All of the ten books have been written and published, the authors being Rick Riordan, Gordon Korman, Peter Lerangis, Jude Watson, Patrick Carman, Jude Watson (again), Peter Lerangis (again), Gordon Korman (again), Linda Sue Park and Margaret Peterson Haddix. Card packs are also being sold which contain 16 different cards that can be used in searching for clues. Each book contains six cards, and the total number of cards is currently 438. Also, its official website offers missions that will lead to clues.
Due to the series' integration of online gaming and card collecting with reading, the series creates an interactive experience. In addition, there are exclusive prizes for children, winnable by buying the books and playing the online game. The novels are designed to incorporate historical elements by placing famous people in the midst of the Cahill family.