By his own admission, the novelist Junot Díaz is an agonizingly slow writer and a chronic procrastinator. Over the past two-plus decades, he has published just three books: two short-story collections and his 2007 novel, “The Brief Wondrous Life of Oscar Wao,” which won the Pulitzer Prize. He once spent about five years working on a 15-page story.

But even by Mr. Díaz’s glacial standards, his latest book, “Islandborn,” is long overdue — about 20 years past deadline. And it’s a mere 48 pages long.

“Islandborn” is a picture book — Mr. Díaz’s first work of fiction for young readers. It grew out of a promise that he made to his goddaughters two decades ago, when they asked him to write a book that featured characters like them, Dominican girls living in the Bronx.

“I’ve always had this over my head,” he said in a recent interview. “They asked me if I could write them something, and foolishly, I said yes.”