Image credit: Shuttershock / Jne Valokuvaus

Walk into a bookstore these days, and you’ll likely see an enormous stack of them: adult coloring books. Sales of the books have skyrocketed in the last couple of years, but their success might hide some depressing news for the publishing industry.

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Quartz has a really intriguing look at the sudden spike in interest in the last couple of years, and the growth is explosive, jumping from a million books in 2014 to 12 million in 2015.

Image Credit: Quartz/Atlas/Thu-Huong Ha

The sudden growth in interest is good for bookstores - in the short run. Fortune reported that sales of physical books rose 2% in 2015, helped mainly by the surge. In the longer run, it’s worrisome, because once interest in the books levels off, that sales boost will vanish.

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If it wasn’t for coloring books, in fact, the printed-book business would likely look substantially weaker than it does. According to a recent ranking at Amazon, five out of the top 10 best-selling books in the United States were adult coloring books.

What does this mean in the long run? Publishing is still struggling: the growth in coloring books makes up for the decline in sales for print books, and that gap is likely going to continue to grow.

[Quartz, Fortune]


Contact the author at andrew.liptak@io9.com.