From an article in today's Wall Street Journal about Barnes & Noble's plan to close stores comes some more information by publishers about e-book sales:
Bertelsmann SE & Co.'s Random House, the world's largest publisher of consumer books, says e-books now make up about 22% of its global sales, up from almost nothing five years ago. The head of a major publishing rival says he expects e-books will be as much as 50% of his total book sales in the U.S. by the end of 2014. Digital books already account for 60% of this publisher's sales of new commercial fiction, a key category for the nation's largest bookstore chain.
Notice how much the percentage of the market that is e-books varies depending on where you sell and what kind of book you sell. Fiction + U.S. = e-books e-books e-books!