According to this week's TIME magazine:by Jackson on 03/13/11
It's a parlor game in publishing circles to guess how long it will take for e-books to constitute a majority of books sold in the U.S. New figures show the answer is soon. The Association of American Publishers has announced growth of 164% in e-book sales from 2009 to 2010--to a total of $441 million in revenue for last year. Some individual publishers have seen even faster growth; Penguin announced that e-book sales nearly tripled in 2010. They still represent only 9% of the book market and 10% of its profits. But with the growth of e-readers like the Kindle and the Nook, those figures will undoubtedly jump. Studies show that consumers with such devices read 2 out of 3 books in digital form.