Author Joan Didion is unveiled as model for ultra-chic fashion label Céline

And that's not the only sign that the literary lioness has a curious cult following...

Joan Didion in the office of her late husband, John Gregory Dunne (AP)

Not since Kim Kardashian balanced a champagne coupe on her generous behind has a single editorial image generated such a frenzied response online.

But this particular picture was an altogether classier affair. This was the latest campaign ad for Céline, the sought-after French fashion label headed by Phoebe Philo, featuring a typically impenetrable Joan Didion. Shot by Juergen Teller, the literary lioness's trademark giant shades are framed by her famous silver bob. She looks, in a word, badass.

The internet went crazy for it when it appeared online on Tuesday evening. In no time, pretty much everyone was proclaiming their adoration for the writer, who celebrated her 80th birthday last month. LOVE magazine stopped reporting on Cara Delevingne's latest activities for five minutes to publish a reading list of Didion's books. The singer Lorde changed her Twitter avatar to the campaign snap. And my entire Instagram feed became clogged with women (and a few men) reposting the image and celebrating this most unlikely pairing.

But how unlikely is it? After all, for a certain slice of the population, Didion's appeal lies not just in her incisive prose but also her timeless chic. She is the original "cool girl".

And Philo has previously hinted at her admiration for the writer. For Céline's resort campaign late last year, she had the model Daria Werbowy recreate a 1968 Julian Wasser portrait of Didion staring out the window of her Corvette Stingray, half-smoked cigarette in hand. So why the fuss?

"Designers are subverting our expectations when it comes to advertising and they're recalibrating the vision of the kind of woman that we aspire to be," suggests Laura Weir, Vogue's fashion features editor. "Joan is an icon. She represents a shift in society that's saying it's not age that matters, it's style that counts."

It might seem odd to celebrate a writer's wardrobe instead of their work, and, at times, the fashion industry has been accused of lionising Didion because she conforms to an aesthetic ideal. But it seems foolish to suggest that one cannot appreciate both her output and her outfits.

Besides, Didion began her career at a fashion magazine. After winning an essay contest in 1956 for which the prize was a job at Vogue, Didion rose from copywriter to features editor and published her first book of essays, Slouching Towards Bethlehem, in 1968. Tom Wolfe was so impressed with her story about a murder in San Bernardino that he included it in The New Journalism.

She has published five novels, a number of screenplays, one play, and more than a dozen collections of non-fiction writing, including her most famous work, 2005's The Year of Magical Thinking, about her grief following the death of her husband of almost 40 years, the writer John Gregory Dunne.

Joan Didion receiving an Honorary Doctorate of Letters at Harvard (Getty)

Now an octogenarian, Didion finds herself as much a part of the zeitgeist as ever. Last October, a trailer for a Joan Didion documentary, We Tell Ourselves Stories In Order To Live, debuted on the crowd-funding website Kickstarter. To be directed by her nephew, the writer and actor Griffin Dunne, it opens with footage of Obama presenting Didion with the National Medal of Arts and Humanities in 2013. "I'm surprised she hadn't already gotten this award," the President says wryly.

With an appeal for $80,000 (£53,000) to make the film, Didion fans pledged $221,135 (£146,700) and it is now in production.

Laura Bailey, the model and writer, confesses to being "long obsessed with Joan". Having contributed funds towards the making of the documentary, she was thrilled to receive a hand-written list from Didion of her all-time favourite books. "2015 reading list sorted," she enthuses. So what is so special about her? "Joan Didion simply makes me want to read more, write more, risk more, and not look back."

Cult following: Joan Didion in 1977 (AP)

Didion's much re-blogged and Tumblr'd packing list (for when she was sent off on assignment at a moment's notice) remains as fascinating as it was when it was first published in her 1979 collection of essays, The White Album ("two skirts, two jerseys or leotards, 1 pullover sweater…"). What makes the list so superb, for many, is her inclusion of cigarettes and bourbon.

So really, the question shouldn't be: why did Céline want to feature the writer; it should be why did the reclusive Didion agree to appear in a fashion ad? At any rate, Céline had better stock up on those oversized shades; the wannabe Joan clones are coming.