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WASHINGTON — When Harry Potter spread his magic through the publishing world, audio books rode on the coattails of his popularity. The audio of version of the series about the young wizard is often credited for helping to raise the profile and boost the sales of all audio books – which totaled $871 million in 2005.

Now audio books are getting another boost from the magic of the digital world. Downloads represented 9 percent of sales in 2005. That's still small, but it's a 50 percent increase over the previous year. Beth Anderson, a senior vice president at Audible.com, says that number will keep going up as more people get MP3 players.

"Part of it is the availability of devices, and part of it is we are so busy, we are looking for ways to do two and three things at once, and to bring reading back into our lives," Anderson says.

The increasing popularity of downloadable audio books is already shaking up the publishing world. Ana Maria Allessi, publisher of HarperMedia, says that in the past, all books came out first in hardcover — but that's changing.

"We're pursuing a number of authors where we are going to work with them on what we are calling 'born digital' products, where we will say it will start its life as a digital download audio book, and may then go to e-book and with that success, we may then go to paper," Allessi said.

As audio-book sales increase, so do the number of digital books loaned by public libraries.

The Montgomery County Public Libraries in Maryland is one of some 4,000 library systems across the country that are now offering digital downloadable books for free. Mary Ellen Ikaza, Electronic Services Librarian at Montgomery County Public Libraries, says audio books have always been popular with library users. Librarians are finding that patrons are eager to switch over from checking out CDs and cassettes because they love the portability and convenience of digital downloads.

"It's available on our Web site, so it's available 24 hours a day, seven days a week," Ikaza said. "So you can be downloading at home in your pajamas at 2 a.m. Or you can be traveling through France, and you decide you want a book to listen to, so you can just log onto your PC and download the book."

As Ikaza demonstrates, downloading a digital book from the public library works pretty much like downloading audio material from a commercial site — except instead of using a credit card, you use a library card.

After choosing from a list of available titles, you click on the book you want and you can audition the book to see whether you like the sound of it.

Some titles are always available. Others might have only a few copies in the library, which means there can be a waiting list for some audio books. Steve Potash is CEO of Overdrive, one of the largest providers of audio books to libraries. He says publishers work out different deals for different titles. But Potash says every download has a copyright protection and can only be listened to for a limited time period — which prevents piracy.

A recent survey of audio-book users indicated that 51 percent of the books consumers listened to came from the library. Ana Maria Allessi says the increasing popularity of downloadable books at the library makes audio-book publishers nervous, because digital books don't have to be replaced; cassette recordings do.

Still, Allessi acknowledges, libraries are helping to bring more attention to audio books, which in the end can only help the business. And right now, she says, there are a lot of unknowns for audio-book publishers. While CDs still represent the largest portion of the audio-book market, Allessi says digital books could overtake them within a few years. And no one can predict how that will affect the business either.