Fortune’s Eugenia Levenson analyzes why freely available and lengthy government reports, such as the 9/11 Commission and Iraq Study Group Reports, are selling like gangbusters. The latter book, published by Random House imprint Vintage, has spent six weeks on The New York Times bestseller list and has already sold over 125,000 copies, according to Nielsen Bookscan, which captures 70 percent of total book sales. That’s despite the fact that the text is freely available online and was downloaded 1.4 million times from a sponsoring Web site in the first week alone.

But it all started with the release of the 9/11 Commission Report, which certainly caught publishing by surprise with 1.5 million copies sold to date. “Everyone was surprised by the retail performance of the 9/11 report,” says Jonathan Burnham, a senior vice president and publisher at HarperCollins. “It seemed to sell to a much broader audience than ever before for that kind of material.” But the real reason for success? The cost, or lack thereof. There are no acquisition costs, since the documents are in the public domain, and are fairly cheap to produce. And because anyone can publish the materials – though the ones who get their first reap the most lucrative rewards – multiple players can be rewarded. “One company shouldn’t have the monopoly on something that’s in the public domain,” says Josh Linsk, CEO of Filiquarian who has sold over 600 copies of the paperback edition. “I just thought there should be more options.”

UPDATE: C.E. Petit points out that the government commission craze began much earlier, with the 1987 Tower Commission Report (released in the aftermath of the Iran-Contra Affair) issued in a mass market paperback edition by Bantam and earlier, in hardcover by Random House.