It is not often that you enter the name of an author you're about to interview into an Internet search engine and come up with her name and the promise of nude pictures. I delicately broached the topic with Claire Harman when we met at her house in Jericho.

She laughed. "You're thinking of the actress," she said. "It is weird to see, because the only Claire Harmans used to be me and a hospital worker in Gloucestershire, just references to catalogues and stuff, very tedious. But suddenly this woman appeared without her top on"

Claire's profession is much more sedate. She writes biographies of writers, having moved into the field after working in publishing, which she fell into after reading English at Manchester University. Her first was on Sylvia Townsend Warner, whose poetry she had edited. Then she chose Fanny Burney and most recently Robert Louis Stevenson.

She felt an immediate attraction for Stevenson, who lived from 1850 to 1894 and is best-known for the novels Dr Jekyll and Mr Hyde and Treasure Island. She said: "He's a fabulous prose writer and charming man and so obviously a charmer," she said.

"He had a charisma which really comes through in his pages and letters when they were published in 1994-95. I'd been very interested in Stevenson before, but when I read them I realised that it completely opened the picture up and made him a man of his time, much saltier and more vulgar and humorous."

He was a contemporary of Thomas Hardy, Joseph Conrad and Henry James, among others. They saw him as their equal although their achievements have generally overshadowed his own, something which Claire discusses in the book.

"He was a very intelligent man and an astonishing stylist who brought finesse to everything," she said. "He's written so many books that people know as classics, though they don't associate them with him as an author, such as Jekyll and Hyde or Treasure Island." She sees it as a mark of his greatness that characters he created have become absorbed into everyday language, although often we don't know that he was the writer.

In later years, as he travelled to warmer climates in search of good health, Stevenson became known for his travel writing, which was very innovative at the time. "That whole concept of being the lone traveller and sending himself up, that's now so popular that it's hard to recognise his achievement," Claire said.

Her book is fascinating, showing as it does Stevenson's struggle with his over-protective, but loving parents to become a writer rather than an engineer and his courtship of the married Fanny Osbourne, whom he followed to California and eventually married. They ended up moving to Samoa, which is where Stevenson ended his days, dying at the relatively young age of 44.

Claire travelled to Samoa, where she visited the house that he had built. His fame in Samoa is predicated on the fact that he lived there, rather than his novels. "He's on their stamps and he's really their only tourist site which isn't a piece of landscape," Claire said.

His poem Requiem, written on the side of his tomb at the top of a mountain, was set to music and has become a national children's song. Claire's book is mainly based on reinterpretation of material already published, but while researching she found some intriguing scraps of paper in the archives, which had barely been mentioned before.

Stevenson had written on these from his sick-bed to preserve his voice. "They're like transcripts of a tape recording," she said. "He's talking about very banal things like what he's going to have for lunch and then parts of responses to conversations about mutual friends and things like that, also about his symptoms, and that's fascinating stuff -- the sort of insight you don't expect to get."

When she is not writing biographies, Claire has been bringing up three children and also teaches a creative writing course on biography half the year at Colombia University's School of the Arts in New York. I asked her what she enjoyed about living between the two places. "New York's the glossiest city in the world, it's magnificent. You see exciting famous people wearing very nice coats and it's got wonderful culture. I think there's something very exciting about the architecture as well," she said.

What about Oxford? She laughs and gets very animated. "When I come back on the Oxford Tube from Heathrow and hit the High, I think: 'Old buildings, bendy roads, clutter'. Oh, it's just great, I love it."

Robert Louis Stevenson is published by HarperCollins at £25.