Short answer - whatever I can lay my hands on. I read fast, anyway.
My guilty pleasures are murder mysteries and historical fiction (preferably not the romances, though).
I just don't get that much pleasure out of trashy fiction, so I don't bother. I've wound up reading some absolute garbage because of Barnes and Noble's free Friday e-books, but I have this problem where when I start a book I absolutely have to finish it (and more than likely if it is a part of a series I have to read the rest of the series) or I go absolutely nutters from not knowing.