An industry trade group reported Thursday that two-thirds of the market for general-interest used books belongs to online sellers. Research by the Book Industry Study Group raised concerns about the viability of traditional used book stores.

Online sales of used books are growing 30% a year, according to the group, while sales at secondhand book stores are flat. "The growth reflects how easy it has become to sell used books," said Jeff Abraham, executive director of BISG, whose members include publishers and libraries. He estimated the value of the used market at $600 million, or roughly 3% of total general book sales.

The Book Industry Study Group (search) believes lower prices are the prime reason for the popularity of used product.

Online sellers including Amazon.com (AMZN) and Alibris.com (search) provided sales figures for the research, despite publishers' criticism. Amazon irritates them by offering new and used books on the same Web page, even on the day of a title's publication. Sources for the used copies can include prepublication author signings and even books distributed to reviewers.

"We're seeing a cultural shift," said Lisa Stevens, marketing director for used book seller Abebooks.com, in a news release some time ago "People don't feel that 'used' is a dirty word anymore. Over 78% of buyers on Abebooks.com say they'd be thrilled to receive a used book as a gift."

The executive director of the Authors Guild (search) worried that rising book prices could lead general interest publishers to follow the practices of textbook publishers, who have also been affected by used book sales and have responded by hiking prices and frequently publishing revised versions, according to the Associated Press.

The founder of the Web's largest open source encyclopedia says the book business is his next challenge. Jimmy Wales of the Wikipedia Foundation (search) started encouraging contributions almost two years ago and the site, at en.wikibooks.org, now has almost 11,500 open content textbooks. Categories include computing, science, study guides, and humanities. "The purpose is to make available to anyone in the world, in any language, a curriculum they can copy, redistribute and modify, for whatever purpose they may have, for free," Wales told Cnet.