The 39 Clues is a series of fantasy adventure books that are published by Scholastic. The first one, The Maze of Bones, written by Rick Riordan, deals with the death of fictional character Grace Cahill. She is the grandmother of the main characters in the series, Amy and Dan Cahill. Grace leaves her relatives a will that invites them to search the globe for 39 Clues that will unlock the secret of the Cahill family's hidden power. The first people that find all 39 Clues are to become the "richest, most influential people in history."

The main part of the series is ten books about Amy and Dan Cahill. They agree to the offer in Grace's will and search for the Clues. The series is unique because the books will be written by different authors. Rick Riordan wrote the first book and the main story arc, but other authors will write the rest of the books in the series. They have been written (in order) by Rick Riordan, Gordan Korman, Peter Lerangis, Jude Watson, Patrick Carman, Jude Watson (again), Peter Lerangis (again), Gordon Korman (again), Linda Sue Park, and Margaret Peterson Haddix.

The series also combines online gaming and card collecting along with the reading into a unique, interactive experience. Six cards come with every book, and more are sold separately in Card Packs. There are also activities related to the 39 Clues online.

The 39 Clues is also unique because readers can search for the 39 Clues themselves. Books, cards and the online game unlock Clues.

There are also prizes with this series for children between the age of six and fourteen. It is possible to win prizes by buying the books and playing the online game.

The books are also designed to teach history. The main characters are fictional, but in this series, almost every famous person belongs to the Cahill family. Because of this, Amy and Dan have to learn about these people to search for the Clues. Therefore, there is a lot of information about history in these book. To get the book 6 clue, get 100 allies.