Now Tim O'Reilly, publisher and founder of tech-book giant O'Reilly Media, has posted a tremendous case study on a book that was available as a free download as well as available in stores. The case study isn't iron-clad proof that giving away books sells books, but it's the best evidence I've seen to date.
The quick answer from this experiment is that we saw no definitive correlation, but there is little sign that the free downloads hurt sales. More than 180,000 copies were downloaded from Jeremy's mirror (which is one of five!), yet the book has still been quite successful, selling almost 19,000 copies in a year and a half. This is quite good for a technical book these days -- the book comes in at #23 on our lifetime-to-date sales list for the "class of 2005" (books published in 2005) despite being released at the end of September. You might argue that the book would have done even better without the downloads, especially given the success of asterisk and the importance of VoIP. But it's also the case that the book is far and away the bestseller in the category, far outperforming books on the same subject from other publishers.Link
Meanwhile, we saw a huge spike in downloads starting at the beginning of this year, but didn't see a corresponding drop in print book sales, other than the continued slow erosion that's typical of books in print (especially one that's heading towards a second edition.) However, we did see the book's first fall from grace, dropping from an average run rate of about a thousand copies a month to about six hundred back in March 2006 coming at about the same time that we start showing the free downloads, but we're not sure whether or not that is just because we don't have earlier download data -- the book should have been available online sooner after publication even though Jeremy didn't start his mirror till March. (Next time we do a book available for free download, we'll be careful to collect accurate data from the start of the project.)