. . . 60% of best-selling electronic books have never entered the monthly top 100 list of best-selling paper books.
. . . 10% of best-selling electronic books correspond to titles that do not exist in paper format . . . electronic books create an opportunity for emerging niche or independent authors to reach their audience as well as to test new markets and price elastiticies.
That is from the new paper Superstars and Outsiders in Online Markets: An Empirical Analysis of Electronic Books by Bounie et al.