The Association of American Publishers shared data from participating StatShot publishers, reporting trade book sales of $620 million for December, down 7 percent. That end-of-year softness stood in contrast to a year in which overall trade sales were strong, growing 4.76 percent on the strength of the adult fiction market and still-rising digital audio sales. For December, adult books sold $457 million, down 6 percent from the previous year. Fiction sold $230 million, falling 3 percent, and nonfiction sold $226 million, down 9 percent. Hardcovers fell 5 percent to $148 million and paperbacks fell 11 percent to $141 million. Ebooks …