How to begin? Begin as she would have begun, you fool. Begin with a sentence that reaches out and arrests; that promises intelligence and mystery; that lures the reader like the sound of distant pan pipes. Because Mary Stewart never started a novel without a line which opens like the door into the rose garden.

Let's revisit some of them. ''Carmel Lacy is the silliest women I know, which is saying a great deal'' (Airs Above the Ground). ''The whole affair began so very quietly'' (Madam, Will You Talk?). ''I am an old man now, but then I was already past my prime when Arthur was crowned king'' (The Crystal Cave). ''My lover came to me on the last night in April, with a message and a warning that sent me home to him'' (Touch Not The Cat). ''I met him in the street called Straight'' (The Gabriel Hounds).

And then there was Wildfire At Midnight, which was the earliest one I remember. ''In the first place, I suppose, it was my parents' fault for giving me a silly name like Gianetta.'' How could any 10-year-old girl stuffed with romantic yearnings resist? Gianetta, of course, was one of the least silly characters you could ever meet: a delightfully cool, intrepid fashion model who found herself caught up in evil on Skye, and whose bravery on precipitous ledges in the Cuillins created pictures in my head which have remained for more than 30 years. Scratch the surface of millions of people middle-aged and older, and you will find the same memories will make a sudden connection with strangers. ''Mary Stewart?'' they say, their faces softening with sudden pleasure, as at news of a dear, forgotten friend. ''You remember her, too? Oh, I loved her books so much.''

Incontestably, Mary Stewart is a great woman; an author of class and verve whose elegant, educated suspense stories founded the whole genre of romantic thrillers. Her work, descended from the mannered prose and suppressed eroticism of Jane Austen and Charlotte Bronte, was arguably what gave birth to the vast world of late twentieth and 21st- century romantic fiction. She built the bridge between classic literature and modern popular fiction. She did it first, and she did it best.

She also produced a fictional trilogy on the Arthurian legend which is compared with T H White's classic work, and which attracted a vast fantasy following; plus a raft of children's books. Her 21 adult novels have never been out of print in the 50 years since the first was published. Fascinatingly, while her public existence as a writer is stellar and enduring, her private life has been subsumed and hidden. At the age of 87, with sales conservatively in excess of five million, Lady Stewart has given so few interviews you could almost count them on the fingers on one hand.

But the things that make her iconic are all in place. Letters written by her are on sale on the internet, along with very rare copies of her 14-page autobiographical pamphlet, written in 1970 in an attempt to stave off the interest in her private life. Meanwhile, the modest creator of one of the best selling oeuvres in the world, widowed two-and-a-half years ago, lives a quiet life in a house by a loch in Argyll. She is described as kindly, impeccably charming, rather daunting, allergic to self-revelation; and still in possession of her fine, robust, classically-trained mind. Somehow, we would expect nothing less.

Kerry Hood, veteran publicist for Hodder & Stoughton, the publishers to whom Mary Stewart has remained loyal throughout her career, regards her great client with awe. She agrees with her place in

literary history. ''The whole romantic genre. I think it's all hers. You think about it. The Moonspinners. The Gabriel Hounds. Airs Above

the Ground. They were fantastic

stories.They stirred the blood of the young heart. It was just such really intelligent stuff, stuffed with literary allusions. They were just such classy books.''

Classy. It is a word you cannot keep away from when you consider either the work or the woman who produced it. Mary Stewart's heroines are all strong, elegant, resourceful women who drive fast cars and leap into action when required. They have a strong moral sense and, could you hear them, would probably sound quite posh. Not for her any silly, clinging creatures with the vapours. In a rare interview about her Arthurian trilogy, she once commented dustily: ''Don't forget what a dreadful life those medieval women must have led, shut up in those ghastly castles while the men were away having fun. Nothing to do but your embroidery, and play at ball in the garden.''

No, embroidery was never an option for Mary's heroines, the

can-do, lovely, well-bred girls who inhabited her books. In many ways, of course, they spoke with the voice of their creator. She usually wrote in the first person. In some ways, we must dare to presume, they were her form of self-expression; the author liberated from her lack of confidence, and freed from her retiring, bookish, middle-class existence, to soar into an imaginary world of adventure and mystery.

She was, above all else, a supreme story teller. The daughter of a County Durham vicar, the Rev Frederick Rainbow, Mary Stewart began writing her first stories before she was five. She was sent to boarding school, where she was badly bullied (for being clever). In the last interview she gave, six years ago, she said: ''The things done . . . are not things one wants to talk about, but it does stay with you all your life. I still have no self-confidence and yet I give the impression that I've got it, because, of course, you are brought up to do that. I can't bear arguments and anger. If someone's unkind, even faintly unkind, you shrivel.''

Mary wanted to be a painter but the family were very poor. Her mother told her that she had to earn a living, so she applied to university. Oxford and Cambridge accepted her, but Durham offered the better bursary. She got a first-class honours in English language and literature and a teaching diploma. During the war, battling poverty, she taught in schools in Middlesbrough. It was on the night after VE day that Miss Rainbow, a tall, slender, pretty woman with blue-grey eyes, met a Scot called Fred Stewart at an impromptu fancy-dress ball. The two married and both became lecturers at Durham University.

They moved to Edinburgh, where her husband became professor emeritus of geology and went on to be one of the foremost scientists of his generation. He was knighted in 1974. Theirs appears to have been a wonderfully happy marriage which lasted for 56 years until his death in 2001. They had shared hobbies of natural history, gardening, Greek and Roman history, music, and art. In later life, with the kind of extravagance she occasionally delighted in, and which her success had granted her, they hired helicopters to drop them off on the tops of Munros.

Their private sadness was their inability to have children. At 30, Mary had an undiagnosed entopic pregnancy. She nearly died from peritonitis. For years afterwards, although she knew it was hopeless, she yearned for a child. Every single month, she has said she thought, ''perhaps this time''. She wanted to adopt but her husband did not.

''I wanted four children. I even had the names chosen. However, there it is. I don't suppose I would have written books if I'd had them,'' she once said, a memorable admission from an intensely private woman.

Out of her personal heartbreak came our pleasure. Madam,Will You Talk?, telling the adventures of Charity Selbourne, was published in 1954. My family copy, dating from 1969, its pages orange with age, has a cover born of its time. ''Charity's gay Provencal holiday turned into something very different . . .'' it says on the blurb on the back. But the story inside is remarkably undated.

Mary Stewart sprinkled intelligence around like stardust. Every chapter was headed with a quote from Marvell or Shakespeare or Browning. The fineness of her mind shone through. Over the next two decades the romantic thrillers came tumbling out: Thunder on the Right, The Moonspinners, Nine Coaches Waiting, My Brother Michael, The Ivy Tree. In how many households were the books passed between mother and daughter, read and

re-read until they fell apart and were replaced with editions with newer, brasher covers?

She had always wanted to write historical novels. One of her great interests was Roman history; she also used to lecture on Anglo-Saxon. In the 1970s, defying her publishers, who wanted her to stay with a proven recipe, she turned to the

legend of Merlin and wrote the Crystal Cave, first in a trilogy with The Hollow Hills, and The Last Enchantment, which reinvigorated her career for a new generation, and, perhaps for the first time, to men.

She has 14 New York Times bestsellers to her credit. In 1968, she was elected fellow of the Royal Society of the Arts. In 1971, the Scottish Chapter of the International PEN Association awarded her the Frederick Niven prize for The Crystal Cave. In 1974, the Scottish Arts Council Award went to Ludo and the Star Horse, a children's book.

Mary Stewart once said that people are either born with it or they weren't. ''You can learn much about the craft of writing, but you either have the storyteller's flair or you don't. It's no virtue of mine. It's just there. In a story, however, each point of rest is also a point of departure; you can't help it,'' she said. Her gift shaped the inner lives of millions of adolescents, their elder sisters, mothers and grandmothers. She was a secret friend to more than she will ever know; her books linger in the memory like old fragrances barely remembered. It is not hyperbole to says she helped make the world an unquantifiably happier place.

All Lady Stewart has asked of her fans in return is that they stop at the gates to her private world. So there we must leave it. Modestly, she is content to be forgotten, but her writing - accessible, learned, spare, full of elan - must not be.

So let me end the way she might like me to, with a quote she used in My Brother Michael.

But enough of tales - I have wept for these things once already. Euripides: Helen