Simon from Bloggasm interviewed some of Tor's authors who've given away their books for free online while they were available in stores and asked them if they believed the giveaways had sold more print books, and what made them think so:
"'Scientifically'?" he [Scalzi] wrote to me in an email. "Probably not, unless you somehow managed to control (or at least account for and factor in) every incident of someone discussing your work and or going down a decision path to acquire the work, which is probably more work than it's worth. But I don't think that 'scientifically' is the standard required here; I think 'heuristically' is probably better. If you consistently see a rise in sales of an author's work after the release of a free e-book, then heuristically you have a good idea it's beneficial."
In his case, Scalzi watched sales of his book shoot up by 20 percent. But what's even more interesting is that the sequel to Old Man's War saw an increase of over 30 percent. Both he and Buckell benefited more from sales of books later in their series.