Scoot over, novels. Coloring books for grown-ups have taken over bookstore displays all over the country. The genre has been around for decades, but in 2013 adults discovered coloring in a big way. Last December, adult coloring books filled nine of the 20 spots on The New York Times bestseller list. In total, adults bought 12 million coloring books for themselves in 2015, and the trend doesn’t seem to be going anywhere.