The next to last free ebook passed out by Tor was Old Mans War by John
Scalzi. He has a lengthy comment on it here with a great number of
comments from readers.
http://scalzi.com/whatever/?p=432
His overall impression is that the free ebook has caused an INCREASE
in sales. Sort of the effect of a 'loss-leader' that brings shoppers
into a supermarket or store and gets them interested in other items
which they will pay for.
That seems to be borne out in many of the comments on his page.
A great number of readers went out and bought the paper book, and some bought others as well.