Harper Lee, who won the Pulitzer Prize for her 1960 novel “To Kill a Mockingbird,” is famous because she wrote one of the greatest of all our Great American Novels. But she fascinates us — the “cult of ‘Mockingbird,’ ” one of my English-major friends calls it — because she never wrote another. And how’s this for a foreign concept: Having said what she had to say, she stopped talking.

Marja Mills tests the limits of that 50-year-plus fascination in her new book, “The Mockingbird Next Door,” which chronicles the 18 months the former Chicago Tribune reporter spent living next door to Lee and her older sister, Alice, in their home town of Monroeville, Ala. Through Mills, a Georgetown grad who met the sisters while on assignment for her paper in 2001, we see Lee guzzling coffee, folding clothes at the laundromat, and pointing out at more length than necessary the unusual names of some of the small towns around there, like Burnt Corn and Smut Eye.

We learn that Lee — who in her private life usually goes by her first name, Nelle — likes fried foods. Before moving into an assisted-living facility a few years ago, she so seldom cooked that she’d been known to use her oven as an extra bookshelf. The small, slow moments for which Mills’s book has been praised certainly are that: By example, Alice Lee inspires Mills to start wiping around the sink after using public restrooms. And both sisters, who succumbed somewhat to modernity by purchasing a television in the late ’90s, repeatedly find it magical that Mills knows how to use the remote and how to print out articles from the Internet.

There are so many of what Mills at one point calls “Oh my God, I’m in an exercise class with Harper Lee” moments that “Look at me, with Harper Lee!” could be the book’s subtitle. Lee has been so low-key and low-tech that the NSA, or even marketers, might have had trouble prying, so you can’t blame Mills for kvelling.

It’s still not clear whether Lee blames her, either, but last week, the “Mockingbird” author’s lawyer issued a statement in her famous client’s name, disputing that Lee ever agreed to cooperate with Mills in any way. Almost at once realizing that the reporter was working on a book about her, “I immediately cut off all contact with Miss Mills, leaving town whenever she headed this way,” the statement said, though unless Mills’s book is 278 pages of fiction — and no one has said it is — that clearly isn’t true. This wasn’t Joe McGinniss moving in next door to Sarah Palin and peering into her yard while penning a takedown.

Author Harper Lee is the subject of “The Mockingbird Next Door.” (AP)

At a reading at Politics and Prose bookstore in Washington on Thursday, Mills answered that accusation by saying that the Lee sisters not only invited her to move in but also served as references with her landlord. She suggested that Harper Lee has had some memory problems since her 2007 stroke. And a spokeswoman for her publisher, Penguin Press, forwarded me a copy of a letter to Mills from Alice Lee in 2011, when the book was announced, that says “Poor Nelle Harper” didn’t realize what she was signing when her lawyer put the original complaint that she hadn’t cooperated in front of her. That lawyer, Tonya Carter at Barnett Bugg Lee & Carter — the old firm of Alice Lee and her father, A.C. Lee — did not immediately return a call seeking comment over the weekend. It was A.C. Lee, of course, on whom Harper Lee based the character of upright Atticus Finch.

But Mills’s own book makes the situation seem a little bit murky, too: Friends of Lee’s warned Mills that the longtime recluse was likely to blow hot and cold on the project, which Mills says she at first thought might be “a book about Mockingbird country, captured as the fictional Maycomb County in the novel” about a black man falsely accused of raping a white woman.

Harper Lee has also taken on the local museum that sells “Mockingbird” memorabilia. Over the course of their long tussle, it wasn’t only that she didn’t like that tacky plastic Madonnas were hawked at Lourdes; she didn’t want the pilgrims to come at all.

Whatever Lee’s agreement with Mills was, there’s no disrespect in the book that came of it. Less than one page in “The Mockingbird Next Door” is devoted to the book’s only negative anecdote, a warning from Lee’s friend and former pastor that she sometimes dials drunk and “accuses people, chews them out,” which Mills soothingly presents as “the flip side to her lust for life. With extraordinary gifts come demons.”

The “setting the record straight” portion of the program is similarly brief: Alice Lee bats down the rumor that her sister’s childhood playmate Truman Capote, who like Mills was also their next-door neighbor, had any role in writing “Mockingbird.” And both sisters take a few whacks at Capote’s insistence that their mother, Frances, twice tried to drown Nelle.

We’re repeatedly told but never shown what a great storyteller and conversationalist Lee is, and the most memorable tale in the book comes from Julia Munnerlyn, a black woman who cooks for the Lee sisters. Before that, Munnerlyn worked as an aide at Monroe County Hospital, where the patients included one of the area’s most dedicated and violent racists. He would run up to her, screaming for her to save him from the black men he imagined were trying to get him. The specter of those he had tormented “ran him to death day and night,” Mills wrote, to the point that “the only way Julia could pacify him was to pretend to kill them.” Which is how a man who’d spent a lifetime hating her people died with his head on her shoulder.

In a sense, we see the same sort of “creating our own hell” in the old age of press- and publicity-hating 88-year-old Harper Lee, who, as Mills points out, had at least a little something to do with launching the “new journalism” style she so abhors when she worked with Capote on “In Cold Blood.” And his nonfiction novel, so described by Capote, was based on the cooperation of subjects with whom the author was most certainly not completely honest.

Mills does ask Lee, obliquely, whether Kansans Herb and Bonnie Clutter weren’t conned out of their privacy: “You were practicing journalism doing that research, wouldn’t you say?”

“There was a difference,’’ Lee insists. Oh? “I knew when to stop.”

The most telling thing Lee says in “The Mockingbird Next Door” might be that she’s afraid of having disappointed a fan who approached her in a restaurant. And Mills nearly does succeed in making her dull through, of all things, overexposure. Not through scandalous revelations, but mundane ones, as though Joyce Maynard had blown the lid off J.D. Salinger’s devotion to flossing.

Maybe that’s because Mills left so much in her notebook: “They . . . wanted a lot of things off the record,’’ she told the heavily female, standing-room-only crowd at Politics and Prose. Yet not all of Lee’s mystery has been stripped away.

At P&P, a man in a T-shirt that read “Spare me the drama” stepped to the mike in the Q&A session and asked with some urgency, “Why did she do it?”

No, not why did Lee write about a false rape accusation — if Mills ever asked her about that, she doesn’t mention it — but why Lee wrote “Mockingbird.”

Because she “had that writerly eye from an early age,” Mills said.

“I’m not sure that’s a good answer,” the man said. But Mills’s book is a good answer to the question of whether Lee was right, all those years, to let “Mockingbird” do the talking.