JANE HARRIS leaves untouched the orange and cardomom biscuit that comes with our tea when we meet on a sunny London afternoon. Perhaps researching and writing her latest, brilliant novel, Sugar Money, about love and innocence, justice and injustice, and sibling rivalry amid the brutality and horror of the 18th-century international slave trade, has put her off sweet stuff for life?

“Ha!” she exclaims. “I don’t have a sweet tooth; I much prefer savoury things.” Which is just as well since the bitter legacy of the savagery of slavery in the West Indies, is made plain in her fine novel. It is Harris’s third. Her two previous historical novels – the runaway bestseller The Observations, with its serving-girl-turned-heroine narrator Bessy’s bawdy, bravura voice, and Gillespie & I, in which a lonely octogenarian looks back beadily on her long life – received rave reviews.

Sugar Money, narrated by Lucien, a slave barely out of his teens and whose voice is rich with the lilting, lyrical rhythms of Caribbean patois, a glorious mix of pidgin English-Scots, French and the kreyol tongue, is being hailed as “superb,” “thrilling” and “a stunning writer.”

In Martinique in December, 1765, Lucien, whose back is “ridged with an island of scars, a map of tyranny,” and his older brother, Emile, “who could out-slink a cat,” are ordered by their French master, Father Cleophas, to return to Grenada, where they once lived and where they were mercilessly beaten by their father. They are to smuggle back 42 slaves who are living under the rule of English and Scottish invaders – “the Goddams” – at a hospital plantation in Fort Royal. The slaves are needed to work in the Martinique monastery’s sugar plantation for rum production.

Both The Observations, a glorious spin on the Victorian sensation novel, and Gillespie & I, about the seamy side of Victoria life, set against the backdrop of the 1988 Glasgow International Exhibition, are works of fiction. As is Sugar Money. It is, however, based on a remarkable true story that 55-year-old Harris found briefly mentioned in a history book when she visited Grenada.

Sipping camomile tea, she explains that when she finished The Observations she embarked immediately on Gillespie & I. She was worn out and had not had a break for ages. She had always felt drawn to “the spice island” Grenada and the French Caribbean but as a struggling writer could never afford the trip – and she had a fear of flying. Following the success of The Observations she booked a holiday and, several hypnotherapy sessions later, set off for Grenada. “I’d had this yearning to go there because it seemed so other -- and it was.”

She spent much of her time exploring and reading about the island. In Beverley Steele’s Grenada, A History of Its People, she came across a brief reference to an expedition that so intrigued her, she knew she had found the subject of her next novel – the story of an enslaved man charged by his masters to steal fellow slaves from the enemy. “I wanted to start immediately,” she admits but had to put the idea aside while she finished Gillespie & I. But then life and several family tragedies intervened.

Always, though, she knew she would write the novel, that it would be the story of a quest and that it would tell of close relationships and not just the lone, historical man who embarked on the original mission. “After writing two novels with solitary, female protagonists, I really wanted Lucien to have someone to bounce off.” The relationship between the brothers is memorable and profoundly affecting – Lucien says, “I found myself too much in simple-hearted awe and adoration of my brother,” while Emile’s heart and soul belong to Celeste, whom he had to leave on Grenada. It is a heart-stopping but hopeful read, and a magnificently challenging one because Harris writes fearlessly of the wicked business of men growing rich out of trading human beings.

“I had to go to some bleak, dark places. It was hard stuff to do, but my other two books also go to dark places. I do like to have some light, too. There has to be hope,” she says. There are, however, harrowing details of horrific punishments meted out – slaves’ ankles weighted with iron shackles, their throats necklaced with spiked collars – while female slaves were abused and raped.

“It all seemed unthinkable to me, such inventive cruelty and just the banality of the evil,” confesses Harris, who has also walked the island with “amazing Grenadian legend, hiking guide and naturalist Telfer Bedeau.” They trekked from St George’s (Fort Royal) to Halifax (Port Havre), following the route of the journey made by her characters, hacking their way in roasting heat through the grasses. “It was difficult but so sensory, and so emotional.”

Many of the cruel “Goddams” Lucien and Emile encounter on their Grenadian journey are Scots for, as Harris remarks, numerous Scots owned Caribbean slave plantations and ran slave ships. And, of course, the obscenity of slavery is still with us. “There are about 40m people still in slavery around the world – in the sex trade, debt bondage, wage labour. Again, it is unthinkable. That for me is the relevance for now of this book, although I never thought of that when I began it. But I did want to raise the notion that Scotland was implicated in the slave trade and raise awareness of that, although I think we’re much more knowledgeable about our involvement now.”

Belfast-born, Glasgow-raised-and-educated, Harris read drama and English at Glasgow University where she was a contemporary of broadcaster Janice Forsyth and actor-director Peter Mullan. Before she became a successful and accomplished novelist, Harris worked as a scriptwriter, actress, and stand-up comic – all three of her books have a richly comic vein. She was living in Portugal years ago when she began writing short stories to amuse herself – mainly about ex-boyfriends – and they were published in various anthologies and magazines.

Back home in Glasgow, she determined on a career as a novelist – her touchstone has always been Robert Louis Stevenson – and gained an MA in creative writing at the University of East Anglia, before becoming writer-in-residence at Durham jail teaching convicted rapists and murderers. In 2000, she received an Arts Council writer’s award and wrote several short films produced by her film director husband, Tom Shankland, from whom she has recently divorced. She has since moved from London to Hastings.

Two films were nominated for Baftas and she was twice listed for the BBC’s Dennis Potter screenwriting award. Then she discovered a long-forgotten series of thematically linked short stories, all set in Scotland, she had buried in the attic of her north-east London flat. There was also an early draft of The Observations. She re-read The Observations and wondered why on earth she had abandoned it and the bold Bessy.

The novel went on to be listed for the 2007 Orange Broadband Prize for fiction and several other prizes. It was later chosen by Richard and Judy as one of their 100 books of the Decade.

Harris writes slowly – “I really am very slow in gestation and it takes its toll. I’m a masochist. I set myself horrendous tasks like writing in Lucien’s voice,” she insists. Still, she loves writing and cares passionately about her work. So passionately, that when she finished the final draft of Sugar Money, she tweeted, “Put a woman on the cover of this, suckers.” She laughs: “I must have been in a bad mood that day.” The tweet was picked up by newspapers.

“If a book is by a woman, they put a woman on the cover. I just thought, ‘what do I have to do not to get a woman put on the cover of this novel told from a male point of view?’ Or, worse, make sure it’s not pink! Then they came up with a garish pink spine for the book and this sort of muted watercolour of tropical seas. It looked like a beach read. Which it is not. My agent and I both said, ‘No way!’ We looked at 16 other covers and I chose the one I liked best – this one,” she says, tapping the gorgeous, abstract, vermillion and black dust-jacket design of sugar cane, which glitters like gold bullion.

Job done? “Definitely!”

Sugar Money, by Jane Harris (Faber, £14.99).