After the lunch, Mr. Sentell said he was asked to take Ms. Lee back to the Meadows, the assisted living facility where she lives. “I’ve never driven so carefully in my life,” he said.

Less than a week before the book’s publication, “Watchman” and the circumstances surrounding its discovery remain shrouded in mystery. The details of the plot have been closely guarded, and little is known about the story, which unfolds 20 years after “Mockingbird” and features the familiar characters Scout and Atticus Finch. The excitement surrounding the publication has been clouded by lingering questions about how and when the manuscript was discovered.

In a taped interview that Mr. Nurnberg, Ms. Lee’s agent, gave to Mr. Sentell before the lunch, he described his shock when Ms. Carter told him last September that she had discovered another manuscript by Ms. Lee. He read it and flew to Alabama to meet with the author. “I told Nelle I found it such a strong novel and I hoped she wanted to publish it, and asked would she like to read it again. She said: ‘Oh no, I remember it very well, no problem. But if you think people will enjoy it, let’s publish it,’ ” Mr. Nurnberg said in the interview, which Mr. Sentell played for The New York Times.

Mr. Nurnberg’s recollection differs with the February news release announcing the publication, in which HarperCollins said that Ms. Lee had been reluctant to release the book until she was reassured by others who had read it.

In the interview, Mr. Nurnberg also described a near-miss moment when, in 2013, he said he held the manuscript of “Watchman” without realizing it during a meeting with Ms. Carter. He said that she had asked him if he would like to see the original “Mockingbird” manuscript. He looked at the first few pages in a large stack, but did not read far enough to discover “Watchman” bundled at the back.

He said that after they left the bank vault where the manuscript was kept, Ms. Carter mentioned that she had detected differences, like characters that were not in “Mockingbird,” and that she suspected Ms. Lee had cut a great deal from the original draft. “I had it in my hand and had no idea,” he said, adding that Ms. Carter did not realize it was an entirely different book, either.

Mr. Nurnberg’s account of the novel’s discovery adds another layer to the evolving story of how the lost book was recovered.