She was reading a letter from one of Georgiana, Duchess of Devonshire's rivals. "I found myself becoming furious on her behalf," says Foreman, whose biography Georgiana, Duchess of Devonshire is published in paperback this month, and who will speak about Georgiana at the British Library on Tuesday.
"She was always treated dismissively by historians," she says. "But her letters were clearly those of an intelligent woman."
Foreman was hooked, and she began a trail of research made more difficult because many of Georgiana's letters were heavily censored by her Victorian descendants. It's easy to see why.
Born a Spencer, Georgiana was a high-rolling, debt-ridden gambler who turned borrowing into an art form.
She was a political activist for the Whigs, while also ruling the social world as the queen of fashionable society.
Privately, Georgiana spent much of her life dependent on the Lady Elizabeth "Bess" Foster, who was devoted to Georgiana. Inconveniently, she also worshipped Georgiana's husband. He once, rather carelessly, got both of them pregnant in the space of a month.
"Georgiana was the first celebrity and Bess was the first fan," says Foreman, of their relationship.
Foreman has achieved celebrity status herself this year, posing nude for a Tatler magazine article, which led some to question whether a serious book was devalued by such publicity.
But, Foreman points out, the nude shot was published months after the book had achieved its success - the hardback has already sold 40,000 copies.
"People were shocked," she says, "but it was fun."
Foreman is now looking ahead to the American publication of Georgiana in January. It should do well: upper-class dissipation in period dress is what they like us for.
Amanda Foreman, British Library, Euston Road, London NW1 (0171-412 7222) Tue, 6.15pm, pounds 5/pounds 3.50
`Georgiana, Duchess of Devonshire' by Amanda Foreman, Flamingo, pounds 8.99