Some people check stock quotations to see how their investments are doing. I look at used-book prices online. I've never been a collector, but over the years a few novels that I've hung on to simply because I liked them have, ah, matured in value. I own a couple of Cormac McCarthy hardcovers from the '80s, for instance, that are now worth $1,500 each. My indifference to collecting took a beating the day I discovered those prices online. But what really holds my attention when I visit a used-book site is how little I have to pay for an out-of-print book that I might just want to read. Used hardcovers that I never hoped to find in a local store are suddenly at my fingertips for $10 or less. If I order promptly, that is, because it turns out that I have competition. Last year Americans spent at least $530 million on used books, roughly 5 percent of the $10 billion trade-book market. In this marginally profitable business, where sales have been virtually flat for the last five years, that's a lot.

Fueled by Internet sales and a growth spurt in the number of used-book stores, this once dusty corner of the book business is suddenly one of its few bright spots. "The Internet made buying used books accessible and acceptable," says Susan Siegel, who with her husband, David, publishes "The Used Book Lover's Guides" and has been collecting data on the used-book trade since 1992. She says that the market, composed mostly of small, independent retailers, has grown more than 20 percent in the past decade. Log on to Amazon, Alibris or Abebooks, visit one of the nation's 7,200 used-book stores, and you can find almost any book, in or out of print. Martin Manley, head of Alibris, nicely sums up the revolution in the used-book trade when he says, "Our tag line used to be 'Books You Thought You'd Never Find.' We destroyed that tag line ourselves."

Manley says Alibris has enjoyed a 50 percent growth rate every year since it started in 1998. It's not unique. Powell's in Portland, Ore., one of the nation's most successful independent bookstores, does 40 percent of its business online; 55 to 65 percent of that volume is used books. And the Internet was merely what got the party started. People who developed a taste for cheaply priced used books sooner or later develop a taste for used-book stores. "Customers like to touch and feel a book before they buy it," says Lorraine Anders, who runs Anders' Attic in Riverside, Calif. "When they go on the Internet, they can't touch the book." According to Ipsos-NPD, a marketing-research firm, used-book buying in stores outpaces Internet purchasing at the rate of about nine to one.

Some used-book dealers say the Internet has helped pick up the slack in a sagging economy. But others refuse to sell online at all. Pulitzer Prize-winning author Larry McMurtry, surely America's most famous used-book dealer, won't go near the Internet. "We sell as well as we do because we're not online," says the Archer City, Texas, bookman. If you want to check out his inventory of 200,000 to 400,000 books, you must travel to the author's hometown store. Plenty of people do, even though Archer City is not near anything, except Wichita Falls, which isn't near anything either. "You put 200,000 books in one place, and people will come," McMurtry says.

The only ones who didn't get an invite to this party are publishers and authors, who make no money off used-book sales. The Authors Guild has been protesting Amazon's selling new and used versions of the same book on the same page. "It may be true that people are buying books that they wouldn't buy otherwise," says Paul Aiken, the guild's executive director, "but people who might be buying new books are being turned into used-book buyers." But don't blame it all on Amazon. As long as used books stay cheap and new books keep getting more expensive, more readers are going to agree with Alibris's Manley that "a used book is just as good as a new book." And in 20 years, that $3 hardcover could be worth some real money.