American book sales were up 18 percent in December. But “despite the supposed good news, the meat and potatoes of the industry, adult hardcover fiction and nonfiction and their paperback counterparts, continue to stagger. Despite robust increases in December — 11.9 percent and 27.7 percent, respectively — sales for both categories were below 2002 marks. What scored the big gains last year were “electronic,” “religious” and “juvenile hardcover,” that last segment’s 28.5 percent rise attributed primarily to “Harry Potter and the Order of the Phoenix.”