Sarah Ferguson says it "really is a dream come true" as her debut adult novel has been named a top ten bestseller.
The Duchess of York, 61, released the book, called Her Heart for a Compass, earlier this month.
It is a fictional account of her great-great-aunt, Lady Margaret Montagu Douglas-Scott, and has been published by Mills & Boon.
According to charts compiled by Nielson Book, the novel is a top ten bestseller in the UK and will appear in The Sunday Times hardback fiction bestseller list this weekend.
Reacting to the news, Sarah said: "I am absolutely delighted to have embarked on a new career as a novelist at the age of 61 and to be rewarded with a top ten bestseller.
"It really is a dream come true and I’d like to thank everyone who has been buying the book so far.
"I hope they are enjoying the exploits of my headstrong, redheaded heroine Lady Margaret and are spotting the parallels between my own life and hers.
"My co-author Marguerite Kaye and I are already hard at work on our second book, which will be set in the same period."
Her Heart for a Compass is described as a "story of duty, family, love, and a woman’s yearning for freedom and independence in a strict, controlling society."
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It promises to sweep the reader "from the drawing rooms of Victoria’s court and the grand country houses of Scotland and Ireland, to the slums of London and the mercantile bustle of 1870’s New York" in the wake of Prince Albert's death.
Sarah explained: "I think people will see the parallels between me and my heroine Lady Margaret – she’s a redhead, she’s strong-willed and she’s led by her heart. She attracts the attention of the press and she makes a career in writing.
"But ultimately, this is a work of fiction and not an autobiography.
"It’s a coming of age story charting the journey of a strong woman who strains against the confines of 1860s society, when women were so controlled. I think the challenges Margaret faces will resonate with today’s readers."
Sarah previously revealed that the idea for the novel came to her after she was approached to appear on hit BBC show Who Do You Think You Are?
She said: "I didn’t end up going ahead with that, but it started me off on a quest to look into the strong, creative Scottish and Irish women in my lineage. I became particularly fascinated with my great-great-aunt, Lady Margaret Montagu Douglas Scott, and the story grew from there.
"What struck me was how small a mark Margaret had made on the history of such an influential family – I could find out almost nothing about her. What kind of person was she? Why had she married so relatively late? She was more or less a blank canvas.
"With real historical events and facts to hand, my imagination took over. I invented a history for her that incorporated real people and events. I was determined to bring her to life and to show that researching and telling the stories of strong women in history of vital importance."
And she also told how she is already in talks for a TV adaptation – and even has her top picks to play her hero.
She told the Mirror: "She says: “We’re talking to some of the streaming services. The leading men would be superb to cast. In my imagination, I see someone like [Poldark star] Aidan Turner, James Norton [of War and Peace] or Tom Hughes [who played Prince Albert in Victoria].”
And for her heroine, the Duchess is all about the fellow redheads: “Lady Margaret would be an actress like Jess Buckley [of The Woman in White] or Eleanor Tomlinson [Poldark].”
