Professor Lawrence Lessig is quoted in a New York Times article on how free online books can increase book sales and do not automatically take away the incentive to buy books:

Lawrence Lessig, a professor at Stanford Law School who has published three of his four books online as well as on paper, said book sales depended on whether the number of people who discover the book online and then buy a hard copy outnumbers “the number of people who would have bought the book and now won’t because it’s free.”