POYKPAC's Coloring for Grown-Ups is back in the headlines after new images were leaked online. These images from the darkly-humorous coloring book poke fun at issues like contraception, sex, and depression. We previously covered the book here.

Adult coloring books are enjoying a boom, and can be found among the top five best-selling books on Amazon. Artists like Kerby Rosanes and Johanna Basford have also created successful coloring books for grownups.

As to why they're popular, Basford has a theory. “It allows us to regress to a mindset when times were carefree and we didn’t have grown-up worries such as making rent payments or an annoying boss at work,” she told Slate. “It’s good to play. You rarely see a stressed out 7-year-old!”

More info: Amazon | coloringforgrownups.com | poykpac.com