I discovered MR with its huge, free and enticing library before I owned an eBook reader. As a result, the offer of a free book from a bookselling website has never gripped me the way free merchandise is supposed to.
One exception: I downloaded a few free things during Read an eBook Week. So far, only one, a short story, has been worthwhile. But that author I plan to buy when his two novels are available as an eBook. So I think that the idea of giving away the first book in the hope that you will buy all the others has merit.