As merchandise manager at Barnes & Noble’s Burlington location, Tracy Moniak has a particularly good vantage point from which to track the ebb and flow of literary trends.

She knows, for instance, that history books do well around Father’s Day. That cookbook sales pick up around the holidays. And that graphic novels are once again gaining steam.

But the recent explosion — and it really is an explosion — in popularity of coloring books for adults? “I did not see this coming,” said Moniak.

In a sudden, unexpected, and generally curious development, grown-up versions of the doodle-books used by countless kindergartners have not only become a thing — but the thing, as far as millions of rapt Americans are concerned.

At the moment, five of the top 30 titles on Amazon’s best-seller list are coloring books aimed at adults. Barnes & Noble currently carries well over 100 different adult coloring book titles, many of which feature much more intricate and detailed designs than children’s versions. And as the trend seeps into the mainstream, publishers and booksellers have been left scrambling to keep the most popular titles on store shelves.

Marketed as a kind of personal therapy session — a simple and solitary alternative to the digital world in which we live — the books seem to have tapped into a deep desire to unwind, unplug, and fend off the stresses of daily life.

“I think it probably speaks to people’s enjoyment in doing this kind of relaxing hobby or distraction from everyday life,” says Sarah Deaver, president of the American Art Therapy Association. “It creates an object of focus, and it creates something that’s beautiful and that’s satisfying.”

The therapeutic effects of art on the human mind have been well documented. There is research suggesting, for instance, that coloring within a premade design may benefit individuals battling anxiety. And many art therapists, though careful to point out that adult coloring and true art therapy are two distinct things, have been quick to throw their support behind the idea.

Yael Assaf-Gruzman, who runs The Muse Art Therapy Studio in Brighton, described the effects as something akin to meditation or yoga — the books serving as a distraction to everyday stresses, while also helping us focus inward.

“For some people,” she says, “I think it’s even spiritual.” 

For a pastime rooted in tranquility, however, the frenzy surrounding adult coloring has been surprisingly zealous.

Evidence of the books’ reach can be found everywher, from the wide-ranging list of titles that fill local bookstore shelves (“Creative Cats,” “Nature Mandalas,” “Hipster Coloring Book”) to the Amazon customer reviews in which fans leave superlative-laced odes to their favorite collections (“BEAUTIFUL!” . . . “GORGEOUS!” . . . “LOVELY,” read three reviews of Johanna Basford’s “Secret Garden: An Inky Treasure Hunt and Coloring Book,” widely considered the granddaddy of adult coloring books).

Sister Charlene Favreau, of Lowell, was at work at Barnes and Noble in Burlington.

It’s there, too, in the adult-coloring events that have begun popping up, public get-togethers in which adult colorers — or “colorists,” as they’re often known — gather with art pencils and crayons to share tips and laud the mental benefits of a good, old-fashioned coloring session.

“It helps me focus on that one moment,” Sister Charlene Favreau, a first-grade teacher at the Immaculate Conception School in Lowell, explained as she attended one such event recently at the Burlington Barnes and Noble. “It helps you to center.”

So great has the response been, in fact, that some publishers were left flat-footed; Basford, whose “Secret Garden” has sold well over a million copies, had to issue an apology on her blog this year after production of two of her books failed to match the demand.

At Brookline Booksmith, phones have been ringing every day with requests for back-ordered titles. On the occasions that one does appear, it’s almost instantly gone, while the section at the front of the store devoted to grown-up coloring supplies currently exists in a state of perpetual disarray.

“It’s just being torn apart every day,” says Lisa Gozashti, assistant manager and art buyer at the store, “by these hungry hordes who are waiting for the next book.”

Though just recently gaining mainstream attention, some versions of adult-aimed coloring books have been around for decades.

Dover Publications has been producing adult-aimed coloring books since the 1970s, according to Ken Katzman, the company’s vice president of marketing.

But it wasn’t until recently that sales began to blossom.

Since 2012, the company’s “Creative Haven” line — currently stocked in stores ranging from Hobby Lobby to Costco — has sold three million copies and emerged as a major player in this new market.

“It’s become literally a new line of business for us,” says Katzman.

In the coming months, Basford is expected to release her next book, “Lost Ocean: An Underwater Adventure and Coloring Book,” through Penguin Random House. Other popular illustrators, including Angela Porter and Marty Noble, are also slated to release new collections of their work. And fans of the mega-hit HBO series “Game of Thrones” were thrown into a momentary state of delirium this month when it was announced that a “Thrones”-themed coloring book would be arriving this fall.

Perhaps the biggest indication of the trend’s stability, however, came recently at Brookline Booksmith, when even the much-anticipated release of a certain sultry novel failed to disrupt the impressive run of the adult coloring book.

Said Gozashti, “They way out-sold the new ‘Fifty Shades of Grey.’ ”