NEW YORK (Agencies) - On Sept 9, the US publisher of "Harry Potter" will premiere a highly ambitious series with a mystery ending for readers and a couple of puzzlers for the industry: How big is the market for a multimedia story - and can a phenomenon be conceived by a publisher rather than created by the public? "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. 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.