Secret Garden and Enchanted Forest, by Johanna Basford, are two of the most popular titles on sale at Amazon.com—and both are coloring books for adults. Featuring detailed black-and-white drawings of the flora and fauna that surround illustrator Basford’s Scottish home, Secret Garden has sold nearly 1.5 million copies.

Fans include Hollywood celebrities such as Zooey Deschanel, and when National Public Radio asked listeners for feedback, many indicated, “I thought I was alone.” The consensus is that adults are seeking to get in touch with their inner child. Beyond the nostalgic charm of coloring books, it’s also a good way for grownups to unwind and reflect.

“So many people have told me that they used to do secret coloring when their kids were in bed,” says Basford. “Now it is socially acceptable, it’s a category of its own.”

For a sample coloring gallery, visit JohannaBasford.com.