In the summer of 2018, Putnam published an unusual debut novel by a retired wildlife biologist named Delia Owens. The book, which had an odd title and didn’t fit neatly into any genre, hardly seemed destined to be a blockbuster, so Putnam printed about 28,000 copies.

It wasn’t nearly enough.

A year and a half later, the novel, “Where the Crawdads Sing,” an absorbing, atmospheric tale about a lonely girl’s coming-of-age in the marshes of North Carolina, has sold more than four and a half million copies. It’s an astonishing trajectory for any debut novelist, much less for a reclusive, 70-year-old scientist, whose previous published works chronicled the decades she spent in the deserts and valleys of Botswana and Zambia, where she studied hyenas, lions and elephants.

As the end of 2019 approaches, “Crawdads” has sold more print copies than any other adult title this year — fiction or nonfiction — according to NPD BookScan, blowing away the combined print sales of new novels by John Grisham, Margaret Atwood and Stephen King. Putnam has returned to the printers nearly 40 times to feed a seemingly bottomless demand for the book. Foreign rights have sold in 41 countries.