The Public Lending Right is a mechanism for distributing funds to authors based on circulation of their books in UK public libraries. It provides an interesting measure of popularity. The PLR has just released the list [pdf] of the highest circulating authors in 2007/2008. James Patterson is now the most circulated author. Before him it was Jacqueline Wilson (now in second place), and before her it was Catherine Cookson, who was enormously popular for many years.
Looking at what these authors write, it's easy to see what people read: mysteries, love stories, children's books, mystery love stories, young adult mysteries, young adult love stories, and the occasional young adult Christmas paranormal love story mystery. [Outgoing]