NEW YORK -- Adult colouring books are giving Harper Lee a run for the money on bestseller lists this summer.

Dover Publications has sold more than 3 million adult colouring books with titles like "Flower Fashion Fantasies." Quarto Publishing will have 1.3 million in print this year ranging from mandalas to fairies. "Secret Garden: An Inky Treasure Hunt," by one of the genre's most popular illustrators, Johanna Basford, remains a top seller on Amazon two years after its initial publication.

In fact, adult colouring books occupied as many of eight of the top 20 slots in a spot-check of Amazon's bestseller list this week, including "Creative Cats" and "Adult Coloring Book: Stress Relieving Patterns."

"We cannot print them fast enough," said Amy Yodanis, Quarto's head of marketing. "We are getting orders of 60,000 at one time from some of our biggest retailers."

There are colouring clubs, colouring contests and a frenzy of colouring posts on social media. Parade magazine devoted a Sunday cover to the trend. Dover plans a national colouring book day on Aug. 2.

"People are stressed and anxious all the time," said Jeannine Dillon, Quarto's publisher. "Colouring is a way to calm down and unwind at the end of the day."

But art therapy is not the only reason colouring has taken off. As hobbies go, colouring books are incredibly simple: portable, easy to pick up and put down, old-school analog pursuits with no batteries or messages, no calorie-counting, skill-building, classes or scores.

And the finished product is perfect for minimalists. Pottery and paintings demand shelf and wall space; knitted scarves cry out to be worn or bestowed as gifts. But a coloured-in page takes up almost no space at all (unless you frame it).

I can attest to the trend's allure. I've been spending my spare moments colouring a book called "Splendid Cities: colour Your Way to Calm."

Not that I've got much to show for my work. It took me more than two months to complete a single page of "Splendid Cities" because I never spent much time on it in one sitting. I'd colour during a stressful moment at the office or at home, or use it as a break from a complicated or boring task, or to transition between tasks.

My longest stretch colouring was an hour while awaiting delivery of time-sensitive documents that I feared were lost. colouring distracted me from worrying about something I couldn't control or fix. I channeled the book's subtitle, "colour Your Way to Calm," and could feel anxious thoughts waning as I concentrated on the picture. Colouring required just enough attention to disrupt the obsessive loop playing in my mind. It wasn't so much relaxation as immersion in something else.

The page I completed depicts a San Francisco streetscape of Victorian row houses with geometric patterns forming gabled roofs and arched windows. I limit my equipment to just four crayons and three coloured pencils, preferring not to complicate my palette with too many choices, and I enjoyed deciding which of my seven colours to fill the template's tiny spaces with. Blue or yellow? Crayon or pencil? Finish the window or start the roof?

When every space was coloured in, I started over, rubbing crayon over pencil, pencil over crayon, mixing colours to make new ones and layering for a mottled effect.

Jason Keyser, 42, a stay-at-home dad from a suburb of Sacramento, California, picked up the hobby a year ago in a program to help him with anxiety and depression after a friend passed away. "I've been doing it ever since," said Keyser, who placed third in a colouring contest for a picture he completed from Dover's "Asian Tattoo Designs."

"It's really relaxing," he said. "Takes your mind away from stressful things in life."