While the series is credited to "Rick Riordan", this is actually a pseudonym.
The actual writers are:
* Peter Lerangis: Wrote books 1, 2, 3, 5, 7, and 9.
* Patrick Carman: Wrote books 4, 6, 8, and 10.
* Jude Watson: Wrote books 11 and 12.
* Gordon Korman: Wrote books 13 and 14.
* Linda Sue Park: Wrote books 15 and 16.
Rick Riordan is the author of the Percy Jackson series, among others, but he did not directly write any of The 39 Clues books. The name "Rick Riordan" was used as a branding technique to leverage the popularity of his other books, especially among young adult readers.