Book publisher Tim O’Reilly writes:

As part of our continued effort to understand the impact on book sales of the availability of free downloads, I wanted to share some data on downloads versus sales of the book Asterisk: The Future of Telephony, by Leif Madsen, Jared Smith, and Jim Van Meggelen, which was released for free download under a Creative Commons license.

Jeremy McNamara of nufone.net, which operates one of the mirrors, provided us with download stats, which we were then able to compare with book sales. Our goal of course, is to help publishers understand whether free downloads help or hurt sales. 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.

Read the whole post with graphs.

Also see one O’Reilly author’s perspective blogged here in January: Linux Kernel in a Nutshell: The Secret Goal.