The Scottish illustrator and self-described “ink evangelist” Johanna Basford may not have invented the adult coloring book category, but she played a major role in turning what was largely a children’s hobby into the publishing craze of 2015. Basford’s first adult coloring book, Secret Garden, was published by the U.K. house Laurence King Publishing and released in the U.S. in March 2013 through its American distributor, Chronicle Books. Sales were solid, but they did not explode until early in 2015, when Enchanted Forest was released. Debra Matsumoto, U.S. marketing manager at Laurence King, says strong word-of-mouth recommendations contributed to growing interest in Garden, and when sales surged in the category, the company was ready with Enchanted Forest. The two titles have not only become mainstays on the U.S. bestsellers list this year, but are hits across the globe. According to Matsumoto, the two titles have combined to sell more than 13 million copies worldwide. Secret Gardenis available in more than 40 languages and has sold almost two million English-language copies and about 7.4 million copies in other languages. Enchanted Forest is currently available in 38 languages, and it has sold almost 1.5 million copies of the English-language edition and 2.3 million foreign-languge copies.

In the U.S., the two titles have sold a total of almost one million copies, according to Nielsen BookScan, which tracks about 80% of unit sales of print books. Sales totals in the U.S. could be higher, but the publisher had some problems keeping the books in stock when demand exploded. (BookScan totals also do not include sales from some nontraditional outlets that have done very well with adult coloring books.) The success of Garden and Forest enticed Penguin Random House to acquire world rights to Basford’s next two books. The first of those titles, Lost Ocean, was published in the U.S. on October 27 and sold more than 157,000 copies in its first four weeks on sale, according to BookScan.

As 2015 draws to a close, it is hard to find a publisher that has not released an adult coloring book. Even J.K. Rowling entered the fray, when her American publisher Scholastic released the Harry Potter Coloring Book in early November. Basford says she began creating adult coloring books as a way for people to relax using pens and pencils rather than fixating on pixels—and given the drumbeat of bad news about economic inequality and the threat of terrorism, it is no wonder adult coloring books are expected to be huge sellers this holiday season.