On a pleasant evening in the spring of 1963, the world’s most famous author felt inspired to write. The pages that came from her typewriter would be saved for decades with a note in blue ink: “Wonderful letter from Nelle Harper Lee.”

It had been three years since the publication of “To Kill a Mockingbird,” and several months since the release of the movie adaptation, when the young Pulitzer Prize winner found herself preoccupied with another deeply Southern topic.

At...