In the world of books, 2015 was the Year of the Adult Coloring Book. How big were adults-wielding-crayons? Big enough that 9 of the top 20 bestsellers on Amazon are now adult coloring books.

That includes the #1 spot, currently held by Adult Coloring Books: A Coloring Book for Adults Featuring Mandalas and Henna Inspired Flowers, Animals, and Paisley Patterns. Fans of our nation's #1 bestseller are enthusiastic. One Top 10 Amazon reviewer gave Adult Coloring Books 5 stars, declaring: Captivating, Complex and Intricately detailed, you will want to color every single picture in this book!

The adult coloring craze, in its current incarnation, has been traced to Great Britain in 2012. Of course, adult coloring books have had their moments before – and there was a tradition of radical ones in the 1960s, skewering everything from the John Birch Society to office life.

Adult coloring quickly leapt over the Atlantic, and has been embraced in the American heartland. The Wall Street Journal reports that public libraries are now hosting BYOB adult coloring book parties and that people are now giving adult coloring books (and Crayola markers) as wedding shower gifts.

How to account for all of this? Adult coloring books are being touted as a stress-reliever and a balm for our anxious times. At least one journalist who tried them out recently became a convert. “While I’m coloring, I let go of my To Do list,” Lindsay Tiger reported in Shape magazine. “I stop thinking about the day ahead.”

The fad has of course attracted its share of skeptics and mockers. The humor site Sad and Useless has come up with its own version, which includes such mature subject matter as a mirror with the instruction: “Draw the person you thought you’d grow up to be before you abandoned all your hopes and dreams!”

For those who find adult coloring books disturbingly regressive, brace yourselves. The Wall Street Journal says the book industry has a hunch where it is all headed next: in January, Little Brown & Co. is publishing its first adult connect-the-dots.