Just as I am about to interview this year’s Women’s prize winner, debut American novelist Virginia Evans, at the party on a drizzly evening in a leafy London square, we are interrupted because someone wants to congratulate her. The fan is Richard Curtis.

A warm-hearted weepy with a sprinkling of gentle humour, Evans’s prize-winning novel The Correspondent is prime Curtis material. In fact, he is too late. “I think he just wants to be my friend,” Evans jokes modestly – Notting Hill is her favourite movie of all time. A film of The Correspondent is already in the pipeline with Jane Fonda playing 73-year-old Sybil Van Antwerp, the crotchety correspondent of the title. Evans will be one of the producers and will have a cameo appearance, “walking a dog or something”.

It is a far cry from when Evans wrote the novel in a closet (she removed her husband’s clothes) over nine months in a rented house in North Carolina, during the pandemic in 2020. She never expected her story, written entirely in letters, of a former legal attorney, to be published, let alone become a word-of-mouth hit, which spent 32 weeks on the New York Times bestseller list.

But the author, who turned 40 earlier this month, is no overnight success. She has been writing for two hours a day, between 5am and 7am, since she was 19, completing seven unpublished novels before The Correspondent. “It is my debut,” she says. “But it doesn’t feel like the first baby, it feels like the eighth baby. It feels as if I’ve always done this.”

Over the years she has received “thousands of rejections” and sent letters to every literary agency in Manhattan “at least once”, she says, before trying those in London and finally finding Canadian agent Hilary McMahon, who recognised that she “had what it takes”. But still The Correspondent wasn’t an easy sell. “It took months, and there was a lot of silence and a lot of ‘nos’,” she says. “It just felt like rejection and ultimately failure was my thing. And it was for a long time – until it wasn’t.”

During this time she did a number of “paycheck jobs” – including working for a lawyer and a surgeon and as a barista – while bringing up her two children, Jack, 13, and Mae, 10, without any childcare. At the point when she moved her desk into the closet, she was contemplating starting law school. But somehow she never gave up. With each rejection, “I felt, ‘OK, I can do better and I have to do better,’” she says. “If you’re a writer, you just can’t not write.”

The novel’s form was inspired by Helene Hanff’s 1970 epistolary memoir 84 Charing Cross Road, which Evans read in a single day during lockdown. She found it so comforting she wished it had lasted longer. So she set about writing a novel in letters that would take in a whole life. John Williams’s 1965 novel Stoner also served as a template of how to turn a seemingly unremarkable life into quietly heartbreaking fiction.

Ornery and outspoken, Sybil, a long-divorced mother of three, is an unlikely heroine in the mould of Elizabeth Strout’s Olive Kitteridge (it is surely no accident that the new paperback cover bears striking resemblances to those of Strout’s). Past tragedy, late romance, betrayal, revenge and gardening club rivalry are all documented in her correspondence: letters to her childhood friend Rosalie, her brother Felix, a troubled teenager, a Syrian refugee, as well as real-life figures including Ann Patchett, Joan Didion and George Lucas.

“I love any book that plays with the format on the page,” Evans says of her decision to tell Sybil’s story in letters. “I think it’s very generous to your reader to give the eye a break. There’s something about letters that feels like a trick. You fly through because the visuals are easier, but the content is not less.”

Despite its warmth and light touch, Evans describes The Correspondent as a book about grief and disappointment. Early in the novel we discover that Sybil’s son Gilbert died many years ago in an accident. While she was writing, the six-year-old son of some very close friends died. Suddenly she felt what it would feel like to lose a child “as closely as I could without it being my own”. When she returned to the book “the echo of his life and the echo of the manner of his death and what it does to a family,” resonated in every passage. She asked her friends’ permission to include Wade in the acknowledgments. “They read it and they said that they would be honoured,” she says. “When the book came out, it wasn’t a big thing. But now it’s all over the world. His mom frequently reaches out to me and says, ‘every time I see the book somewhere, I just think that these people also now know of his existence.’ So that is really one of the best things about this success.”

Maggie O’Farrell has said that she delayed writing her former Women’s prize winner and now Oscar-winning film Hamnet, about the death of Shakespeare’s only son from the plague, until her own son was safely past the age at which he died. Evans took the opposite approach – and made Gilbert eight, the same age as her own son Jack at the time she was writing. She listened to an interview with Zadie Smith in which the novelist said of the maxim that you should write what you know, that you should also write what you fear, because they are equally vivid in your mind. “I realised that it’s so true,” she says. “I could only write that grief accurately by trying to get as close to the thing as I could.”

One of three siblings, Evans grew up in Maryland. It wasn’t a particularly bookish household. But, like Sybil, she has always written letters, especially to authors she admires. Ann Patchett became a pen pal and is now a friend and supporter of the novel. Evans had a little uneasiness about the imaginary letters from Didion and Larry McMurty included in the novel. Both authors replied to fanmail and she was careful to make sure they were based on things they had written. “I love to receive a letter,” she says. “It’s like an artefact. I have some letters that are real treasures.” Now she is inundated with letters and has to have help to reply to them all.

For all its sadness, she wanted the novel to have an “uplift”, she says gesturing with her hands. “A lot of books, you get to the end and you feel, ‘Oh dear, this is very, very bleak.’” She thinks this hopefulness may account for why the novel has struck such a chord, especially today. Redemption is unfashionable in fiction, she admits, which she worried might count against The Correspondent. “It says something really beautiful to me that so many people were willing to entertain my book. A book about hope and a book about forgiveness and a book about grief and disappointment. That those things are so valued makes me feel quite optimistic.”

Its success means she can finally write full-time, although she sticks to two or three-hour stretches once the children have gone to school. And she has come out of the closet: she now has a room of her own – “a little porch”. She is well into a new novel, about making a movie. But she still can’t quite believe in her own triumph. Recently she asked her agent: “’Do you think this thing will sell?’ She laughed at me, said, ‘Yeah, now it will sell. Everything will sell.’”

• The Correspondent by Virginia Evans (Penguin Books, £9.99). To support the Guardian, order your copy at guardianbookshop.com. Delivery charges may apply.