The cover is creased and the edges slightly curling, but otherwise The Memory Keeper's Daughter is in surprisingly good condition for a book that has travelled more than 1,000 miles and been through seven pairs of hands.

"I don't bother about creases on the cover or the spine," says Wendy Evans, 48, the seventh and current owner of Kim Edwards' novel, which has travelled from Preston to Leicester via Glasgow and Dorset before landing on her doorstep in Sheffield. "I do object to food residue, but this one's in pretty good nick."

Evans has exchanged 135 books through ReadItSwapIt.co.uk since last August. "It's addictive," she says. "I can try out authors I wouldn't normally read and I don't feel guilty if I give up halfway. I'm not paying for the book, and I'm not throwing it away after I've read it or leaving it to gather dust on a shelf."

For eco-aware readers, the environmental benefits of swapping rather than buying are clear. In 2003, Greenpeace launched its book campaign, producing evidence that the UK publishing industry was inadvertently fuelling the destruction of ancient forests in Finland and Canada. It found that one Canadian spruce produces just 24 books, which means that if you get through one book every two weeks your reading habits destroy almost one large tree every year. (In the same year, Greenpeace persuaded Raincoat Books to produce the Canadian edition of Harry Potter and the Order of the Phoenix on recycled paper, saving an estimated 39,000 trees.) But despite the campaign, only 40% of the UK book industry has introduced paper with a high level of recycled content, largely choosing to use paper certified by the Forest Stewardship Council instead.

Beyond using the country's dwindling network of libraries, until recently the opportunities for exchanging paperbacks have been limited to friends, community schemes and book groups. But in the past two years, a spate of online book-swapping sites have emerged. Inspired by the goodwill schemes operated by hostels around the world, whereby travellers can leave behind books they have read and pick up something new, these sites generate little profit for their founders. The books are swapped directly between users, who pay the postage; the sites simply facilitate the meeting and identifying of potential exchanges.

On BookMooch.com, a site run from California, users enter the titles of the books they want to give away, and earn credit that enables them to borrow each time they swap a book. "I was inspired by a community centre I saw on holiday in Norwich," says founder John Buckman. "It had a bookshelf outside with a sign saying, 'Leave a book, take a book'. I liked the idea of them circulating around the world."

What sets BookMooch apart from sites such as WhatsOnMyBookshelf, PaperBackSwap and Bookins, is its international scale: it has 68,930 users in 91 countries. Since its launch in 2006, nearly 700,000 books have been swapped; The Memory Keeper's Daughter, the most exchanged - or "mooched" - book, has been swapped 755 times. Edwards' tale is something of an online sleeper hit, beating bestsellers such as Khaled Hosseini's The Kite Runner and Ian McEwan's On Chesil Beach to the top spot on both BookMooch and ReadItSwapIt.

Earlier this year, Philip Felstead, 57, from Dorset, was the fifth person to read the well-thumbed copy that now sits on Evans's bedside table. "It's not a book I would have bought," admits Felstead, whose job involves publishing of a different kind: monographs by retired diplomats. "But I read two or three books a week in the evenings, and I was looking for something fun." Felstead, who has been using the site for 18 months, passed the book on to Teresa in Leicester, who in return sent him Engleby by Sebastian Faulks. "Extremely interesting," is his verdict. "I've been introduced to many authors I'd never have tried."

Some readers include a note - just a friendly hello, or a recommendation - creating a sense of community and continuity. Some even meet up to swap books and discuss them in person, says Martin Bathgate, founder of ReadItSwapIt.co.uk. "Our forum is a great social tool. People who like books tend to like each other."

Evans also welcomes the social aspect of book-swapping. An administrator at the church army training college in Sheffield, she used to rely on church book sales or the local library, "but they didn't always have the most recent books. Getting hold of new novels and talking to people about them makes a real difference".

Of course, those churches and charity shops that made money from second-hand book sales stand to lose out, as do the publishing industry and authors. "In the music industry, this kind of thing would be called 'file sharing', and technically illegal," the author Jeanette Winterson wrote of book-swapping sites recently.

"Of course I want people to read my books, but I also want people to buy my books."

But Buckman and Bathgate argue that the sites function alongside libraries and bookshops to provide access to a wider range of titles, encouraging readers to encounter new authors and keeping older titles in print. "People who use the site become fans of books and end up buying more," says Buckman. "One in 20 of the books exchanged is also purchased."

Despite working in publishing himself, Felstead is philosophical about the impact on the industry. "I suppose one doesn't worry so much about the bookshops these days, because they're all large chains that make most of their money selling coffee. And authors should be pleased that their work is being disseminated around the world."

The Memory Keeper's Daughter, meanwhile, is about to go to its next home. At first, Evans says, she felt uncomfortable about sending her books to strangers. "I was brought up to take care of my books," she explains. "But at least I know it's going to a good home, to someone who wants it, not to sit on a shelf or be thrown away".

· This article was amended on Thursday May 1 2008. In the original version we didn't introduce Andrew Bathgate in full, and also referred to Wendy Evans as 'he', not 'she'. This has been corrected.

· This article was amended on Monday May 5 2008. We incorrectly amended the article to include Martin Bathgate's first name as Andrew. This has been corrected.