MONROEVILLE, Ala. — One morning late last summer, Tonja B. Carter was doing some legal work for her prized client, Harper Lee, when she found herself thumbing through an old manuscript of what she assumed was “To Kill a Mockingbird.” The characters were familiar, as they would be to millions of readers — the crusading lawyer, Atticus Finch, and his feisty daughter, Scout. But the passages were different. Atticus was much older. Scout was grown up. The story unfolded in Alabama during the racial turmoil of the 1950s, not the Depression of the 1930s.

Confused, Ms. Carter scanned the text, trying to figure out what she was holding. It was a novel titled “Go Set a Watchman.” It may be one of the most monumental discoveries in contemporary American literature.

“I was so stunned. At the time, I didn’t know if it was finished,” Ms. Carter recalled in an interview on Saturday, her first extensive comments about the discovery. She went to see Ms. Lee and asked her if the novel was complete. “She said: ‘Complete? I guess so. It was the parent of “Mockingbird.” ’ ”

The recovered manuscript has ignited fierce debate — much of it speculative — about why Ms. Lee waited so long to publish again, whether the book will stand up to her beloved first novel, and whether the author, who has long shied away from public attention, might have been pressured or manipulated into publishing it.