The 39 Clues

This is a mystery adventure series that is written by numerous authors. It is also an unusual series because it invites the reader to uncover clues along with the book characters.
Rick Riordan (author of The Lightning Thief) wrote the first book and there is a collaboration of six other authors in a series that has a total of nine books. Authors that have contributed to the series include Rick Riordan, Gordon Korman, Peter Lerangis, Jude Watson, Patrick Carman, Linda Sue Park, Margaret Peterson Haddix, Roland Smith, and David Baldacci.