The one and only

"Maycomb was an old town, but it was a tired old town when I first knew it. In rainy weather the streets turned to red slop; grass grew on the sidewalks, the courthouse sagged in the square. Somehow, it was hotter then: a black dog suffered on a summer's day; bony mules hitched to Hoover carts flicked flies in the sweltering shade of the live oaks on the square. Men's stiff collars wilted by nine in the morning. Ladies bathed before noon, after their three o'clock naps, and by nightfall were like soft teacakes with frostings of sweat and sweet talcum."

That's how Harper Lee described Monroeville in 1958 when she first started writing To Kill a Mockingbird. That's how it's fixed in the imagination of more than 30 million people throughout the world who bought copies of the book after it came out two years later - that one stifling summer in a small town, when a six-year-old tomboy called Scout slowly learned not to be frightened by her strange neighbour, Boo Radley, and when her father, Atticus, saved an innocent black man from the electric chair.

That one book is, however, all we have from Harper Lee. All the awards - the Pulitzer for the book, the four Oscars for the film version starring Gregory Peck as Atticus Finch - all the editors' enticements, all the publishers' pleas, have failed to push her to attempt another book. To Kill a Mockingbird will, she seems to have decided long ago, be the only book with her name on its spine.

Nelle Harper Lee turns 80 today. She still lives in Monroeville, a town of only 7,000 souls (who, because this is deep Bible Belt country, have 28 churches to choose between). She shares an unostentatious one-story redbrick house near the town's courthouse with her sister, Alice Finch Lee, who, at the age of 94, still practises law. Neither of them ever married.

Yet, even though Harper Lee hasn't given an interview since 1964, interest in her has seldom been higher. Of course, in a celebrity-fuelled age, her very reticence has that effect. But two major Hollywood movies look set to make it even harder for her to sidestep the spotlight.

The first, Capote, for which Philip Seymour Hoffman won this year's Best Actor Oscar for his performance in the title role, highlighted a fact that is almost as unusual as Lee's status as a one-book wonder. It's this: that in one small dusty town in Alabama in 1931, 500 miles from the nearest city of any size, two world-class writers grew up next door to each other. More than that, they were lifelong friends, so that, as the film shows, when Truman Capote travelled to Kansas in 1959 to investigate the murders he wrote about in In Cold Blood, the researcher he chose to help him was none other than Lee herself. She'd just finished her novel and while she waited for it to be published, she was at a bit of a loose end.

Later this year, another film, Infamous, in which Lee is played by Sandra Bullock, will go over almost exactly the same ground. It, too, will show how heavily the camp, diminutive Capote relied on Lee during their first two months interviewing people about the Kansas murders.

"She is a gifted woman," he once told his biographer, George Plimpton, "courageous and with a warmth that instantly kindles most people, however suspicious or dour. She was extremely helpful in the beginning, when we weren't making much headway with the townspeople, by making friends with the wives of the people I wanted to meet, especially the churchgoers." But if In Cold Blood - "the world's first non-fiction novel", as Capote called it - might not have got off the ground without Lee (to whom it is dedicated), would To Kill a Mockingbird ever have been written without Capote? In it, he appears as Scout's boyfriend, Dill, "a pocket Merlin whose head teemed with eccentric plans, strange longings and quaint fancies".

For most of his childhood's summers, Capote lived next door to Lee, at the house of an aunt who had a milliner's shop in Monroeville. Abandoned by his mother, who dumped him with her relatives while she went off to lead a high social life in New York, the young Capote became friends with the Lees.

Capote was notoriously unreliable when talking about his past, and his claims to have taught Lee how to write (when he was eight, she was six) should be treated with caution. At the time, he recalled, she wanted to be a lawyer like her father, and was quite happy to type up the stories he dictated to her in her father's office at the back of the house. Another of his anecdotes has him writing a story about an eccentric local recluse, "Old Mr Busybody" for the Mobile Press-Register, only for his outraged aunt to visit the newspaper and snatch it back before it was published. The inference that he was the first to write about a character suspiciously like the reclusive Boo Rad ley in To Kill a Mockingbird may well, however, be sour grapes from a writer whose ego was wounded when his friend beat him to the Pulitzer in 1962.

Inevitably, in view of Harper Lee's enduring reticence about talking about her work, there has always been a residual suspicion that Capote was the real writer of To Kill a Mockingbird. "He absolutely was not involved," her sister says. "That's the biggest lie ever told."

Yet to describe Harper Lee herself as a recluse is way off the mark. Unlike JD Salinger, for example, she never deliberately cut herself off from the world or bought guard dogs to protect her from a prying public. She might turn down requests to watch Monroeville's amateur dramatic society's annual production of To Kill a Mockingbird, she might object to local plans for a Harper Lee Day, or veto a planned "Calpurnia's Cookbook" full of recipes of the kind her fictional family's black servant would have cooked for them. But, in all other respects, her life is barely affected by fame.

For half the year, she lives in a one-bedroom apartment in Manhattan, where she regularly attends concerts in the Lincoln Center and elsewhere, and takes relatives to Mets baseball games, invariably using public transport, on which she is seldom, if ever, recognised. Occasionally she even attends public events at which she is the guest of honour - as she did last year for an awards ceremony to which she was invited by the widow of her friend, Gregory Peck, who deservedly won an Oscar for playing Atticus Finch in To Kill a Mockingbird.

Back in Monroeville, where she spends the winter months, she and her sister have none of the luxuries her wealth would allow her. There isn't even a computer in the house. "All she needs is a good bed, a bathroom and a typewriter," says a friend. "Books are the only things she really cares about." And, although she can often be heard tapping away at her typewriter, there's never been any hint that she's doing anything more than politely turning down interview requests. Asked why she's never written another book, in the past she has replied that, after Mockingbird, anything else would inevitably be an anti-climax. But why, after all these years, does that one book hold such enormous and enduring appeal?

For part of the answer, you can go right back to the start, to that description of Maycomb in the mid-1930s. Its cadenced, simple yet elegant sentences take us straight back into a childhood that is vividly realised yet isn't suffocated by nostalgia.

All the school study packs (for this is a set text for many of Scotland's 14-year-olds) will tell you that this is a story about racial bigotry and injustice informed by the Scottsboro Boys trials, when nine black men were accused of raping two white women in Alabama. And they're right: as Atticus Finch tells his daughter: "As you grow older, you'll see white men cheat black men every day of your life, but let me tell you something and don't you forget it - whenever a white man does that to a black man, no matter who he is, how rich he is, or how fine a family he comes from, that white man is trash."

But there's something else, too, something in the cultural ether that gives us two Hollywood films at roughly the same time that both use Harper Lee's straightforwardness as a foil for Capote's moral ambiguity and self-aggrandising cynicism. She's not quite Atticus Finch, that great shining example of gritty American moral courage and human decency, but she's the next best thing.

And in these dark days - every bit as overshadowed by the imminence of terror and barbarism Scout and Jem were taught about in their school at Maycomb in the mid-1930s - we need reminders of moral grace more than ever. Both in her life and in her one wonderful novel, Harper Lee has provided us with them.