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Today, the U.S. publisher of the "Harry Potter" books premieres 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. 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.

"The word we always used was 'groundbreaking,' " says Scholastic executive editorial director David Levithan. "We wanted to be the first out there to introduce this kind of multidimensional thing."

A Scholastic team, led by Levithan and including about a dozen editors, thought of the series about three years ago, working from the idea of a treasure hunt. The essential outline, including the ending, was set by the publisher. Authors were asked to fill in the details, taking a thread, as Levithan describes it, and turning it into a blanket.

"It's a different kind of challenge," Levithan says. "To use a movie analogy, each director of the 'Harry Potter' films brings their own voice and their own vision to what J.K. Rowling has done. You still feel there's a consistency there, and part of the fun is seeing what they add to it."

Scholastic quickly decided that "The 39 Clues," its title an homage to Alfred Hitchcock's "The 39 Steps," would make an ideal multiplatform event. Readers might check out the Web site, just as kids who love online games might then turn to the books. A recent study by the American Library Association revealed that many librarians already use games to attract young people and, ideally, get them interested in books.

"I love the gaming aspect of 'The 39 Clues,' " says Jenny Levine, a digital specialist for the library association. "I could also see a lot of libraries forming '39 Clues' clubs the way they've had Pokemon clubs."

Books for all ages often originate with publishers, and countless best-sellers are made through marketing. But a blockbuster, whether "Harry Potter" or Stephenie Meyer's "Twilight" novels, virtually always happens spontaneously.

"Harry Potter" was born in the brain of Rowling and immortalized by millions worldwide. The staff at Scholastic, and the British publisher, Bloomsbury, were sure they had a hit, even a classic, but not a record breaker. Other children's franchises, including "Clifford" and "Junie B. Jones," began simply as books and expanded only in response to public demand.

"I remember when 'The Sisterhood of the Traveling Pants' first came out; nobody knew it was going to be so big. That's how it works. You need the kids to grab onto a book and tell each other about it," says Beth Puffer, manager of the Bank Street Bookstore, based in New York.

"I can't think of a phenomenon that was presented that way from the start. This is a very unique situation."

Puffer and other booksellers are enthusiastic about "39 Clues," although unsure whether it will be a sensation. Kimberly Diehm, co-owner of the Neverending Story Children's Bookshoppe in Las Vegas, calls the first volume "a perfect tale" by Riordan, but says she has noticed little discussion about it among her fellow retailers.

Becky Anderson, co-owner of the Anderson Bookstores in suburban Chicago, says she is a little wary of the project's ambitions: "It was presented to us as the thing that's going to replace 'Harry Potter.' " But she was "blown away" by "The 39 Clues."

"We're investing big in this," she says. "I think we see it as a way to get some of those nonreaders into reading."

Other multimedia projects are being developed. HarperCollins is working with former Scholastic executive Lisa Holton on an eight-book series for girls. Dutton, a division of Penguin Group (USA), recently acquired a mystery trilogy by "C.S.I." creator Anthony Zuiker that will be complemented by an interactive Web site. Simon & Schuster will release "Spaceheadz," Internet sites and a series of chapter books co-authored by Jon Scieszka and Francesco Sedita.

"In the past we've made the mistake of demonizing other media, saying all TV is bad, all computers are bad, and all books are good," says Scieszka, appointed last year by the Library of Congress as the National Ambassador of Young People's Literature. "Kids know that it's not true; there is great television and there are great games.

"I just also want to make sure that we don't forget what's unique about a book, losing yourself in an extended narrative."

"I think it will be fascinating to find out if this is a trend that we'll be seeing a lot more of," says Dutton senior editor Ben Sevier, who added that he and other publishers would be watching how the public reacts to "The 39 Clues."

"It's hard to manufacture a phenomenon," he says of the series. "It's an enormous risk, and it signals an enormous enthusiasm."