When John Buettner of Chestertown, Md., wants to buy books, he heads to a used bookstore. The price is right, he says, and "there is a certain 'hunting' appeal to the whole process."

George Simpson of New York takes a different approach: He goes to his computer and shops for used books online. A few clicks of the mouse, a few taps of the keys, and his order is complete.

Both men represent enthusiastic customers in one of the fastest-growing segments of the book industry: used books. Last year, US consumers spent more than $2.2 billion to buy 111 million used books. That's up 11 percent from 2003, according to a landmark study by the Book Industry Study Group. Most significantly, the growth is happening on the Internet. Online sales of used trade books - those aimed at a general audience - jumped 33 percent last year. By contrast, trade-book sales at traditional bookstores rose 4.6 percent.

"Used trade books have been growing 25 percent a year," says Jeff Hayes, group director of InfoTrends/CAP Ventures, a research firm in Weymouth, Mass. "New trade books are growing only a couple percent a year."

As these "pre-owned" volumes become an increasingly popular choice for millions of readers, they are shaking off their once-dusty image. They are also beginning to shake up an industry that until now has paid little attention to them. Publishers and authors worry that they will siphon off sales from new books. Others see their rising allure as part of an emerging shift in how Americans view ownership.

"The used-book market is just beginning to burgeon," says Dan Nissanoff, author of the forthcoming book "Futureshop." "Culturally, buying used books as a way to acquire a book is becoming mainstream and acceptable." He traces the shift to an "auction culture" that "promotes temporary ownership as an efficient way to live a better life.

"We've historically been a society that accumulates, that views acquisition as permanent," Mr. Nissanoff continues. "Now channels such as Amazon.com, eBay, and Barnes & Noble have made it easy for us not only to buy books but sell our books as well. They're accelerating the auction culture and pushing this behavior to the mainstream. It's beginning to have a big impact on the book industry."

Buyers of used books span all ages and demographic groups. Less than 3 percent of consumers buy only used books, Mr. Hayes says. About 30 percent purchase new books exclusively. The majority buy both.

"The more avid readers are much more likely to buy both new and used," Hayes says. "Those who buy only a few titles a year buy new."

Adding to the appeal of used books is the increasing availability of "gently used" books in excellent condition, often for sale soon after publication. Referring to the books he purchases online, Mr. Simpson says, "I have never gotten one that wasn't nearly new in condition."

Stephen Andrews, book review editor of the Journal of American History, explains the appeal that secondhand titles hold for him. "A used book has more value than a new one because it has passed through someone's hands, acquired notation and comments, and become a living thing. I'm less interested in books that are in pristine condition."

Mr. Andrews's wife, Elizabeth, describes him as an "avid buyer of used books, with packages arriving almost every day from Amazon."

Hardcovers account for the majority of sales of used books, Hayes says. Yet Rich Fitzpatrick, owner of the Braintree (Mass.) Book Rack, finds the market for used hardcovers "very weak." He sells most used hardcovers for a dollar, but used paperbacks are priced at $3 or $4. Customers like the portability of paperbacks, he says.

For customers, the widespread availability of used books offers many advantages. "The Internet creates a whole new market and almost a whole new pricing," says Richard Davies, a spokesman for online bookseller Abebooks.com. "You have a series of prices lined up side by side," which gives buyers choices.

One issue the study does not answer is the impact that used book sales are having on new book sales.

"That remains an open question," says Jeff Abraham, executive director of the Book Industry Study Group. "Some people might argue that it cannibalizes new book sales. Some argue that it spurs new book sales by making more books available and by bringing them to new locations."

Whatever the answer to that question, not every customer is lured by the siren call of Internet bookselling.

"To me, roaming along the shelves of a used book store is like searching for seashells along the beach," says Michael Robertson of Charleston, S.C. "Sometimes I leave the store with a full pail, other times I do not."

Booksellers like Mr. Fitzpatrick also remain optimistic that there will always be bricks-and-mortar stores, and room for a variety of ways of marketing the printed word.

"We're doing great," he says, although he notes that two used-book exchanges in nearby suburbs have closed within the past month. "There are a lot of doomsayers. But my attitude is, for the last 400 years a lot of people have been making a good living selling books, and I think that will continue."