Everyone from Walmart to Barnes & Noble is looking to cater to a booming group that see coloring as a way to unwind in a digital world.

A quick search on YouTube reveals hundreds of adult coloring book reviews, tutorials and tips. In fact, four out of the top ten selling books on Amazon are adult coloring books.

The trend of adult coloring has taken off this summer. Proponents of the hobby find it to be a way to wind down, relax and wash away the rigors of everyday life.

Amazon released its list of the 20 best-selling books for 2015, and for the first time two coloring books for adults are on it.