Sales for adult coloring books have had another increase by at least 20 percent in the last few weeks. A 9 percent increase in demand under the non-fiction adult category was boosted by the adult coloring trend during the week ending Aug. 16, according to Publisher's Weekly.

Among the most-coveted books, Johanna Bashford's "Secret Garden" continues to top book sales. "Secret Garden" sold 36,500 copies in last week alone. To date, the book has over 1.4 million copies sold, with 28 different editions released in the foreign market. Bashford is due to release her third coloring book, "Lost Ocean," in October.

The second most popular adult coloring book dominating sales this August is "Creative Cats Coloring Book" by Marjorie Sarnat, which has sold more than 8,000 copies. It currently ranks 11th in overall book sales for the week ending Aug. 16.

In 12th place is "Color Me Calm" by Lacy Mucklow and Angela Porter. The book has sold 7,900 copies in the past week. A third book from the pair, "Color Me Stress Free," is due for release in mid-September.

New comer "Fantastic Cities: A Coloring Book" from Steve McDonald was released Aug. 11 and sold 7,100 copies in its debut week. It is in the 19th place in the top 20 best-selling overall book ranking.

"We keep thinking it's going to end, but it's getting bigger and bigger," said Jeannine Dillon, who works for Quatro, the company that publishes the "Color Me" series, according to Miami Herald. "When we first published last October, we went with modest numbers, and the first printing was sold out. We reprinted, then had to reprint again."

Sales for adult coloring books peaked this spring. "We've been selling them for a long time, but the big spike really started earlier this year. I think it caught most publishers by surprise," says Ken Fund, the CEO of Quarto Publishing Group USA, according to Mercury News.

"I think a huge part of what's making them so popular is that it's such an absorbing, relaxing activity," said Millie Marotta, the illustrator of "Animal Kingdom" to the same news outlet. "It's easily accessible. You can spend five minutes or five hours, depending on your schedule. It's an engaging activity that makes you focus your mind on what you're doing, so it's easy to switch off your mind from everyday worry and stress. ... We all did it as children, so it's familiar and comfortable."

Marotta, a native of England, is currently working on the release of her still-untitled third book, according to BBC.