Neil Gaiman and his publisher have published the results of their free online release of his novel American Gods earlier this year — the conclusion? Giving away ebooks for free sold books:
The Indies [ie. independent booksellers — Neil] are the only sales channel where we have confidence that incremental sales were driven by this promotion. In the Bookscan data reported for Independents we see a marked increase in weekly sales across all of Neil's books, not just American Gods during the time of the contest and promotion. Following the promotion, sales returned to pre-promotion levels.