Harper Collins has analyzed the data, and concluded that their one month free access to Neil Gaiman’s American Gods had direct and measurable increase on sales.  They were only able to measure it among independent booksellers, as there was an alternate Gaiman promotion going on at the big box bookstores:

 The Browse Inside Full Access promotion of American Gods drove 85 thousand visitors to our site to view 3.8 Million pages of the book (an average of 46 pages per person). On average, visitors spent over 15 minutes reading the book.

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.

Gaiman’s Journal for full post