Sex, coloring and mystery: This is what adults go in for in 2015, according to Amazon.com Inc.’s list of best-selling books for the year.

While the appeal of sex and mystery are no, well, mystery, adult coloring books may be a surprise to many. Two books in that category made Amazon’s AMZN, -4.12% list—the first time any coloring book has been among the best-selling books.