A bestseller is not a successful book. It is a measurement. When two million people buy the same novel in the same decade, they are not all responding to the prose style, or the plot, or the cover. They are responding to something the book supplies that ordinary life, in that particular decade, is withholding. The sales figure is evidence of a specific collective hunger. And if you know how to read it, it tells you more about a society than most official records.