FOR all her fame, Harper Lee has remained something of an enigma. A shilling life will tell you most of what you need to know about her. Born in 1926 in Monroeville, Alabama, she died in the same, small place ninety years later. She attended local schools and the University of Alabama. After graduating she spent a while in New York, a city she loved, working in the reservations department of an international airline. She liked to travel, including to Scotland, read, and listen to eighteenth-century music. In general, she preferred the company of cats to most people. She never married.

One of her childhood friends was Truman Capote, whom she helped research – if not write – his celebrated “non-fiction novel”, In Cold Blood, about the killings of four members of the Clutter family on a farm near Holcombe, Kansas, in 1959. Lee herself, of course, is synonymous with To Kill a Mocking Bird, published in 1960, which to this day is the one novel most Americans have read if they read at all. A commercial as well as a critical success, it encouraged many journalists to make the trip to Monroeville in the hope of interviewing Lee, virtually all of whom departed with nothing to show for their trouble.

Like JD Salinger, she wanted to be left alone and felt no need to explain her novel or to open up her life to strangers. She was not reclusive, as Wayne Flynt testifies in Mockingbird Songs; she simply did not welcome uninvited guests. Those who did managed to get through her front door and got to know her a little wrote at length about her. One such was Marja Mills, a journalist and ostensible friend, who insisted that Lee had asked her to write her “authorised biography”, which turned out not to be the case. As Flynt writes, Mills’ book, The Mockingbird Next Door, consists of a few well-told anecdotes, “and lots of information about Mills herself”.

Flynt, in contrast, is no muckraker, nor is he a hearse chaser. An Alabama academic specialising in the southern state’s history, he first got to know the Lee family in the 1980s. There were three sisters; golf-loving Louise; Alice, who was fascinated by history; and Nelle, as Harper was known to friends and family, the youngest of the three. In the beginning communication with Lee was irregular and formal. She and Flynt did not meet often but kept in touch by letter. There was a decade, he recalls, between her first letter to him and her second. Once, she sent him and his wife a Charles Rennie Mackintosh postcard from “her precious and dwindling stock” bought from Glasgow’s Hunterian Gallery “just because she wanted us to have one”.

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The letters from her in this short book are naturally charming, spikily witty and devoid of pretension. Ahead of one visit she tells Flynt not to expect anything other to eat than catfish, “which is about all you can get in Monroeville”. Furthermore, she adds, it’s best not to visit on Monday or a Saturday when there is nothing at all to eat. She clearly liked Flynt, perhaps, he speculates, because history, not literature, was his subject. He was also a Baptist and an ordained minister, which may also have been a reason why they gelled. Lee offers few words about her work. What, she wonders, would the response To Kill a Mockingbird have been had she made it “complex, sour, unsentimental” as opposed to simple, sweet and sentimental, as some critics see it. It would, she reckons, have received “great critical acclaim” and never had a second printing.

Lee was in her sixties when her correspondence with Flynt, who was in his fifties, began. They were mutually respectful, though his awe of her sometimes grates. As the years pass, age takes its vindictive toll. The belated publication in 2015 of Go Set a Watchman, a prequel to To Kill a Mockingbird, in which Atticus Finch, the hero of the latter, is revealed as a racist, sent yet more journalists to the Deep South in search of the elusive author. None got anywhere near her.

Seven months later, Nelle Harper Lee died. At her funeral, as she had requested, the eulogy was delivered by Professor Flynt. Why, he asked, does Mockingbird go on selling nearly a million copies a year half a century after it was first published? Quite simply because what happened to an innocent black man in a fictional place called Maycomb County, Alabama, could happen anywhere at any time, “to the different, the strange, the other”.

Mockingbird Songs: My Friendship With Harper Lee; by Wayne Flynt is published by William Heinemann, priced £9.99