How many books in the 39 clues series did Rick Riordan write?

Rick Riordan wrote one book in the 39 Clues series:

* The Maze of Bones (2008) - This is the first book in the series, and it was the only one written solely by Riordan.

He also wrote the final book in the series, The Serpent's Shadow (2010), but this was co-written with Peter Lerangis.