It may surprise fans of Johanna Basford’s intricately hand-drawn coloring books that the artist is, by her own admission, “pretty bad” at coloring.

“I can’t stay in the lines,” she said sheepishly.

Not that it matters. Ms. Basford’s coloring book “Secret Garden,” a 96-page collection of elaborate black-and-white ink drawings of flowers, leaves, trees and birds, has become a global best-seller.

Since its release in spring 2013, “Secret Garden” has sold more than 1.4 million copies in 22 languages. It shot to the top of Amazon’s best-seller list this month, overtaking books by authors like Harper Lee, Anthony Doerr and Paula Hawkins. Her follow-up, “Enchanted Forest,” which came out in February, is briskly selling through its first print run of nearly 226,000 copies.

What makes Ms. Basford’s breakout success all the more surprising is her target audience: adults who like coloring books.