John Hilton III and David Wiley have published "The Short-Term Influence of Free Digital Versions of Books on Print Sales" in the latest issue of The Journal of Electronic Publishing.
Here's an excerpt:
Increasingly, authors and publishers are freely distributing their books electronically to increase the visibility of their work. A vital question for those with a commercial stake in selling books is, "What happens to book sales if digital versions are given away?" We used BookScan sales data for four categories of books (a total of 41 books) for which we could identify the date when the free digital versions of the books were made available to determine whether the free version affected print sales. We analyzed the data on book sales for the eight weeks before and after the free versions were available. Three of the four categories of books had increased sales after the free books were distributed. We discuss the implications and limitations of these results.
