I've seen them everywhere (and not just bookstores!). We're talking offline businesses like doctors, therapists, etc....
Well guess what. For us baby boomers:
The Lisa Frank Adult Coloring Book Has Arrived -- And It's Only $3
There's some neat resources to cashing in yourself:
How To Self-Publish An Adult Coloring Book With Meg Cowley | The Creative Penn
Artist cashes in on adult coloring book craze - Tech Insider
This Company Turns Your Instagram Posts Into Coloring Book Pages
Have you added this to your business?