(CNN) Atop the Amazon bestselling books list this month sat an unexpected title: "Secret Garden." It wasn't Frances Hodgson Burnett's 1911 novel about a sour little girl's magical place, making a book club comeback.
It was a similarly named coloring book that adults were buying, for themselves, and it wasn't the only one in the top 10.
Johanna Basford's "Secret Garden: An Inky Treasure Hunt & Coloring Book" (now at No. 3 on Amazon) along with her second effort, "Enchanted Forest: An Inky Quest & Coloring Book" (No. 6); "Balance (Angie's Extreme Stress Menders Volume 1)" by Angie Grace (No. 9); and "The Mindfulness Colouring Book: Anti-stress Art Therapy for Busy People" by Emma Farrarons (No. 8 on Amazon UK) are selling at a rapid clip.
Though they can be used by kids, these and other new coloring book titles are being marketed to stressed-out, work-addled adults, who want to benefit from the quiet zen that a coloring session can bring.
"Adult coloring is absolutely a growing trend and consumers are really taking to the idea," Farrarons' U.S. publisher, Matthew Lore of The Experiment publishing group, wrote in an email. "Not only is it calming and good for your health, it's just fun! The demand is increasing exponentially as the word spreads."
While Farrarons and Basford are based in the UK, the concept is taking off in the U.S. too, with the publication of titles like Virginia-based art therapist Lacy Mucklow and illustrator Angela Porter's "Color Me Calm" and "Color Me Happy," created for the minds and motor skills of Mom and Dad, not the kids.
The trend doesn't seem to be letting up. Basford is working on a third title, Farrarons has been commissioned for a second book, and Mucklow and Porter will release "Color Me Stress-Free" in September.
Adults have long used crafts to unwind, but why coloring books? Why now? It may have something to do with online access -- and, funnily enough, the desire to unplug.
Ordering a coloring book that suits adult tastes online is easier than walking into a bookstore where the only options have Barbie or Thomas the Tank Engine themes. Plus, everyone's favorite online crafting hub, Pinterest, is a treasure trove of adult coloring pages, with themes ranging from nature and animals to classic paintings.
Meanwhile, like children, adults need a break from screen time -- and many are rediscovering the analog pleasures of coloring inside the lines.
"I'm a grown-up, but I still love coloring books," novelist Matt Cain proclaimed in a piece for The Guardian.
"If I switch off the phone, computer and TV and concentrate solely on choosing the right shade of blue, avoiding going over the lines and slowly filling up my page with colour, all my other concerns, I've discovered, fade to nothing," Cain wrote.
The therapeutic benefits of art are nothing new; it's a concept that practitioners use with patients of all ages.
Atlanta-based art therapist Susanne Fincher, who has published several coloring books, said coloring can lift the mood, reduce anxiety and relieve stress.
"Art making is a powerful intervention," Fincher wrote in an email. "Neuroscientific research has shown that through the use of art therapy, the human brain can physically change, grow, and rejuvenate."
True art therapy, she warned, should be administered only by a qualified professional.
Mindfulness and meditative coloring are recurring themes in the growing adult coloring book industry. A search for "adult coloring books" on Amazon or Barnes and Noble will yield several books of mandalas, a ritual symbol in Buddhism and Hinduism that represents the universe, waiting to be colored in.
"I sometimes give clients one of my mandala coloring books for homework between sessions with me," Fincher wrote. "Coloring mandalas can empower a client to manage thoughts and feelings on their own with the positive activity of coloring, instead of, for example, overeating or abusing substances."
The opportunity to craft a mindfulness coloring book for adults was serendipitous for illustrator Farrarons, who had been practicing mindfulness for a few years before getting the offer to create a book.
"In mindfulness, it is encouraged to break patterns in life by introducing variation to avoid the sensation of being on autopilot," Farrarons wrote in an email while on holiday in Korea. "Each page has been caringly designed with this in mind, so that the person coloring can hop at random from one pattern to the next. "
Coloring books like Farrarons' pocket-sized volume bring a bit of calming and spirituality to the masses, but for some adult enthusiasts coloring is just a fun throwback to a simpler time, she added.
"It reminds me of hours spent filling in scenes from a coloring book as a little girl. In the digital age that we are in, surely, it can only be a good thing to pick up a pencil and feel young again."