Once upon a time, Rick Riordan was a San Antonio author known for his Tres Navarre detective thrillers (like The Last King of Texas and Big Red Tequila) before he hit it big with his Percy Jackson and the Olympians series of children’s books, involving the re-telling of classic myths. The best known one remains the first, The Lightning Thief.

But now Riordan (pronounced RY-er-dun) has a much, much bigger deal in the works, involving the U.S. publisher of Harry Potter, a 10-part book series and an online treasure hunt:

The 39 Clues is a planned 10-volume set about young Amy and Dan Cahill and their worldwide search for the secret to their family’s power. The first book, The Maze of Bones, is written by Rick Riordan of The Lightning Thief fame and has an announced first printing of 500,000. Steven Spielberg has already acquired film rights to the series.

Designed for boys and girls ages 8 to 12, each book will have a different writer, including such best-sellers as Gordon Korman and Jude Watson. Backed by a multimillion-dollar marketing campaign, The 39 Clues also features game cards, a contest with a $10,000 first prize, and a sophisticated Web site that includes games, blogs, videos and thousands of pages of background.

Here’s the AP story and here’s the NYTimes.