I don't know, but while gathering material for a mildly snarky reply I was surprised to learn that romance novels (often thinly-disguised porn) are probably the biggest-selling e-book category, followed by SF and fantasy. Readers like escapism during a recession, and e-books are cheap enough to be a guilt-free discretionary purchase.
http://www.teleread.org/2009/04/08/romance-novels-are-big-in...