11 books in series

The 39 Clues

The 39 Clues is a 11-book series with 11 released primary works first released in 4 with contributions by Rick Riordan, Gordon Korman, Peter Lerangis, Jude Watson, Patrick Carman, Linda Sue Park, and Mallory Kass.

The Maze of Bones
One False Note
The Sword Thief
Beyond the Grave
The Black Circle
The 39 Clues: In Too Deep
The Viper's Nest
The Emperor's Code
Storm Warning
Vespers Rising
The Black Book of Buried Secrets