According to Publishers Weekly:
Fiction is the leading sales driver of e-books, and just how dominant that segment has been is clearly seen in a new market study just released by Bowker. According to “2010–2011 U.S. Book Consumer Demographics & Buying Behaviors Annual Review,” fiction accounted for 61% of unit sales in 2010 and 51% of revenue, putting it far ahead of other segments. The children’s segment generated 12% of unit sales in 2010, placing it second behind fiction, while nonfiction accounted for the second highest amount of e-book revenue in 2010, with the segment representing 14% of sales.
Another indication of e-books’ importance in the fiction market is the format’s share of unit sales among genres. Of the 10 genres where e-books represented a significant portion of sales in 2010, only one, biography/autobiography, was nonfiction. The genre where e-books accounted for the highest portion of unit sales was literary/classics, where e-books generated 20% of unit sales last year. The high percentage reflects the large number of public domain titles of classic books being sold through a variety of Web sites.
More details, including charts, in the article.
Thanks to Michael von Glahn for the link.