Avid readers everywhere can identify with the challenges Phyllis Gatto used to face in finding space for books she had read. After finishing a paperback, she would put it on a shelf. When the shelves filled, she moved books into cartons, hiding them under beds and in closets.

"The pile would just grow," says Mrs. Gatto, of Dayton, Tenn. "I'd give some to friends, but basically, they just accumulated."

Then a friend told her about an unusual book-sharing website, PaperBackSwap.com. Members swap used books, paying only the cost of postage - usually $1.59. In addition to saving money and freeing space, members can make electronic connections with far-flung readers.

Similar online book-trading sites in the United States include FrugalReader.com and TitleTrader.com. A British website is ReadItSwapIt.co.uk.

At PaperBackSwap.com, members list at least nine paperbacks, earning three credits. Credits allow them to search available titles and choose up to three. Senders pay the postage. They receive one credit for each book they mail, enabling them to order other titles. The website formats a mailing wrapper. The sender then prints out the wrapper, adds stamps, and mails the book.

"It's very user-friendly," Gatto says.

The venture grew out of founder Richard Pickering's years as a business traveler. "Traveling through so many airports, you accumulate a lot of books," he says. "I had no easy way of doing anything with them at home. They collected on my bookshelf."

He tried selling them on eBay and Amazon.com, but grew tired of fees. Determined to find a better way to trade books, he teamed up with partner Robert Swarthout, who had developed a college textbook exchange.

They now list more than 300,000 paperbacks, along with audio books. Because books are sent by media mail, membership is limited to the US, Guam, and Puerto Rico.

Participants include a range of ages and demographics, with women outnumbering men. Mr. Pickering describes a typical member as a woman who loves to read, is bored by TV, and owns many paperbacks. Some members post as few as 10 titles. Others have posted more than 1,000 and have traded hundreds. In all, members mail from 1,500 to 2,000 books a day.

Romance novels and mystery thrillers are the most popular genres. Other favorite categories include home and gardening, home schooling, and Christian books - "anything you can imagine," Pickering says.

Megan Aepli of Boston, who joined PaperBackSwap about nine months ago, reads about eight books a month. "I'm constantly reading three books at a time," she says. "That's why this is great for me."

Gatto orders both books and audio tapes, listening to the latter while traveling. "I like historical novels for reading," she says. "I don't normally read mysteries, but I listen to them." She has listed nearly 60 titles, shrinking the literary cache under her bed.

Books must be in "good" condition. A few readers, however, have complained that a book doesn't fit that description. "It's a legitimate concern in some cases," Pickering says. "We try to put it in perspective and say, 'They were free.' " He estimates that one or two books out of 100 might not live up to the proper rating.

Other features on these sites include readers' reviews, a wish list of titles not listed, discussion forums, and chat rooms.

Gene McCabe, who is president of FrugalReader.com, says that his site's credo is "Read, share, relate." Beyond helping people save money, he says, "It's about helping them connect with each other. They love to read, but they also love to talk about that reading experience, talk about the books, plots, authors."

Such connections can have far-reaching effects. After hurricane Katrina, members in the discussion forum at PaperBackSwap.com decided to donate books to the ravaged area. They found a county in Mississippi where two libraries had been destroyed. "Our members mailed boxes and boxes of books from all over the country to help rebuild the libraries," Pickering says. Librarians have asked them to stop sending because their warehouse is full.

The sites are free, but modest membership fees are ahead. At PaperBackSwap.com, Pickering expects fees to be $10 or $20 a year. McCabe, too, says, "At some point we will have to recover costs and get a revenue stream."

ReadItSwapIt.co.uk encourages reading by giving everyone in the United Kingdom access to free books, says Andrew Bathgate, cofounder.

Environmental charities have praised the site for recycling books. Libraries in London and elsewhere have also promoted it. Traffic peaked after Christmas, Mr. Bathgate says, when recipients of unwanted gift books began "swapping like crazy."

Book-sharing websites help people who can't get out or who have little access to books. "In a lot of places in rural America, you might have to go 100 miles for a bookstore," says Susan Siegel of Book Hunter Press in Yorktown Heights, N.Y., who has studied the used-book market. "For them, it's a fantastic outlet."

No figures track the impact of the Internet on sales of used paperbacks, so no one can say whether cyberspace swaps will siphon business from bricks-and-mortar bookstores. "There's certainly a potential for that," Pickering says. "But we cater to a different market that wants ease of use. They don't have to get dressed and drive down to the local used-book store."

Dena Russ, assistant manager of B & L Books in Altamonte Springs, Fla., which offers a book exchange, says that swapping online has not affected her store. "Die-hard people who really love bookstores are never going to get out of the habit. They like the atmosphere. They like to go in, peruse the books, smell the books, hold them."

Readers like Ms. Aepli remain enthusiastic about cyber-trading. Recalling the books that once collected dust in her apartment, she gives the website a succinct, thumbs-up review: "It works really well."