“Despite occasional hot sellers such as ”The Da Vinci Code,” book sales have grown little in the past few years, and mass-market paperback sales have declined steadily. One big reason, it seems, is that baby boomers, historically the biggest mass-market buyers, increasingly find those little books too hard to read. Mass-markets are the thick, squareish paperbacks — mostly entertainment fiction — that you stick in your pocket or purse and read on the subway, airplane, or beach. It’s a tried-and-true format. But something is wrong.”