Nearly a decade ago, Eileen McNally caught a National Public Radio interview with an obscure writer named J.K. Rowling. The book being discussed, "Harry Potter and the Sorcerer's Stone," sounded like fun, so McNally picked it up for her 9-year-old niece.

"I bought it for her to read on the plane home to Buffalo," said McNally, now director of the Florida Center for the Book at the Broward County Library. "But Shannon read the entire thing standing in line at Disney World. I was flabbergasted. That's when I knew this was something special."

Special, indeed. The six Harry Potter books published since 1997 have so far sold more than 325 million copies in 65 languages. They've spawned a blockbuster movie franchise and a merchandising empire, and made Rowling, by some reports, richer than the Queen of England.

Scholastic, Rowling's American publisher, reports the sixth volume, "Harry Potter and the Half-Blood Prince," sold a stratospheric 6.9 million copies in its first 24 hours, making it the fastest-selling book in history.

The seventh and final installment, "Harry Potter and the Deathly Hallows," hit shelves yesterday. Advance orders on http://Amazon.com topped 1.6 million, another record.

Those kind of figures have made publishers and booksellers increasingly dependent on Rowling to bump up summer traffic and boost year-end profits.

Last year, according to the trade association Publishers Study Group, total U.S. book sales were flat in the absence of a new Potter book, rising only .05 percent over 2005.

This year the study group projects growth of 6.5 percent, almost entirely the result of "Deathly Hallows."

For Sarah Silberman, a self-described "obsessive" Harry Potter fan in Broward County, Fla., the last book brings mixed feelings. "I'm really excited, but I'm also all sad and depressed," said Silberman, 13. "This is the last one. This thing you've been waiting for through all these books will be over. The reality of that hasn't sunk in for me."

Publishers and booksellers likewise mourning the end of Harry Potter are anxious to find something to take his place. Their hopes lie with such other young-adult fantasists as Cornelia Funke ("The Thief Lord"), Eoin Colfer ("Artemis Fowl"), Michael Scott ("The Alchemyst"), or possibly nonfantasy novelists like Brian Selznick ("The Invention of Hugo Cabret") or Meg Cabot ("The Princess Diaries"), whose new series, "Allie Finkle's Rules for Girls," debuts in February.

The problem: No one can explain what exactly made Harry so phenomenal.

"There's no science to this," said Mitchell Kaplan, co-founder of the Miami Book Fair. "I'm sure writers are hard at work right now trying to figure out how J.K. Rowling came up with one of the most successful creations in literary history. They'll fail."

The next big thing, Kaplan added, will be "someone just writing a great story. It's always about the story."

Largely because of Harry Potter's sales dominance, The New York Times created a children's best-sellers list, which Lisa Holton, president of trade publishing for Scholastic, said is "a great marketing vehicle" for all genres of juvenile books.

"What's interesting about Harry is the effect we see not only on Scholastic but on everyone," she said. "It's gotten more kids reading, it's invigorated fantasy as a category, and it's benefited other categories as well."

Beverly Horowitz, publisher at rival Delacorte Press, readily agreed. "The difference is that a children's book this successful is bigger than its category," Horowitz said. "Kids are going into the store, but so are parents and grandparents. It's a big event, and we use that time to push other books."

Since Harry Potter, said Holton, publishers have been "surprised at the depth of interest in fantasy books, both ours and others'. It's been incredible how deep the interest in fantasy goes."

That doesn't mean the next big publishing phenomenon will automatically be a fantasy title, Holton added.

"Kids are fickle and a little edgy," Horowitz said. " 'I'm bored, what's next.' We're not trying to be Harry Potter, but we are trying to capture that kind of energy, the energy of a kid being interested. You can't replicate the experience of Harry Potter. You have to offer them something new."