Paperback sales for previous titles by Ms. Clark and Ms. Grafton have stayed similarly consistent. "As each of these authors has a new novel, they may dilute" their own hardcover sales, Mr. Kirshbaum said. "But when you take all their books together, they may actually be growing." Ms. Grafton, he said, may be selling less of her latest mystery, "Q is for Quarry," in hardcover, "but she's still selling A through P."