On a cold Friday morning in February 2022, Cory Leadbeater took his final walk with Joan Didion. It was down Madison Avenue on Manhattan’s Upper East Side, where she and he had walked “thousands of times”, an odd-couple pairing well known to the nearby museums and restaurants.

Didion was a literary legend who, in her eighties, was in body (but not in influence), vanishing almost to nothing. Leadbeater, her longtime live-in companion and carer, was a burly man in his twenties who had secrets and struggles of his own. This well-trodden walk took them back to the vast apartment that the millions of readers of her last great work, The Year of Magical Thinking, will know as the setting of Didion’s exquisitely excruciating grief for her husband and daughter.

“But the appreciable difference that morning,” Leadbeater writes in The Uptown Local, his memoir of Didion’s last years, “was that Joan had been dead two months by then, and I was carrying home her ashes.”

This book is in a way a coda to The Year of Magical Thinking, the 2005 best-seller that turned Didion’s unsentimentally acute reportage on herself. Didion, who made her name as the author of classics such as Slouching Towards Bethlehem and The White Album, wrote about the year in which her husband, John Gregory Dunne, died, and her daughter, Quintana, was dying.

Just as Didion described in that book, she continued to keep her apartment as it was when they were alive, isolated in a kind of twilight of memory and longing. But then a change was forced.

In her late seventies she needed someone to live in, not providing care like a nurse but managing her affairs. In 2013, ten years after Dunne’s death, Didion finally remodeled her home, to accommodate a bedroom for the arrival of Leadbeater, an aspiring young writer. He was a graduate student at Columbia University when James Fenton, the British poet and one of his tutors, connected him with his friend Didion.

It marked a shift in both directions: her decline, and something new — cohabiting with young life in her otherwise greying circle of Manhattan’s high literary society. Didion latched on to Leadbeater. At one point, early on, she fell. As Leadbeater helped her, she “gripped my hand and made me promise not to leave her; I did promise, I did not leave her”.

Leadbeater was there for the fun: the 5pm ritual of cigarettes and wine; their kitchen table debates about authors and artists; the high-octane dinners with Supreme Court justices and movie stars; the Netflix documentary; he helped to arrange for Didion to become the face of Celine in 2015.

But it was also Leadbeater, a “lonely and lower middle-class” boy from unfashionable New Jersey, somehow who was the one to sit in constant vigil with Didion during two hospital stays; he was there when she told him in that first hospital crisis that she didn’t want to go on, “I’d rather not be, than be like this.”

A few days later Didion, recovering, put her hand on his neck and apologized. For years he hid from her his past at the hands of a physically abusive father who was later jailed for fraud, and his despair and thoughts of suicide. When Leadbeater in turn wanted to end it all, Didion unknowingly saved him, because he felt he could not break his pact to look after her until the end. In nine years the most they were separated for was a few days.

“I’d rather not be, than be like this.”

He was there when Didion died. He was also there long after she died, paid by the estate to visit her apartment every day for a further year and a half. Just as Didion had preserved that apartment in the magical belief it would preserve something of her husband, so Leadbeater stayed locked in his routine of visiting that same apartment, unable fully to admit she was gone. Didion had prevented him from dying, but was she now also preventing him from living?

Leadbeater, 35, has a shaved head and full beard, a look part-hipster, part-hoodlum, when we meet by video call from his home in New Jersey. He is warm and self-deprecating, torn between his anxiety not to violate the privacy of his great hero while also eager to talk of her as much as he can.

“Joan’s world offered me an emergency exit, a trapdoor into a wildly free, safe new life,” he writes, but was he free? He shows me the alarm screen on his phone. Three and a half years on it still blares out the schedule of his life with Didion, five alarms marking his breakfast duties onwards. He can’t bring himself to turn them off even when they drive Liz, his girlfriend, crazy, and wake up their baby daughter from her nap.

When they ring he says, “I love you Joan” to no one. I peer at them as he holds his screen up: the one at 3.30pm is marked “dinner”. Why so early? In the book he writes that she loved her food, she “was built like a bird her entire life, but had the palate of a glutton”.

By the time they discussed what she wanted and ordered out, the pair would be ready to eat at 5pm. Didion would end each hungrily consumed take-out meal with vanilla ice cream, eaten with a fork (he couldn’t bear to throw it out of her freezer), and a second glass of wine. Why can’t he turn the alarms off?

“It’s ridiculous. I just can’t bring myself to. I know it drives people in my life insane, but I don’t want to turn it off,” he says. “My heart breaks for Liz sometimes. She just accepted that the center of my world was Joan, not our relationship. When Joan needed me, I went.”

Leadbeater of course became more than an assistant: at one point he describes, self-critically, “living as if married to a woman in her eighties, making no progress on my own work and dining instead on the greatness of my boss”.

At another point a stranger asks Didion if Leadbeater is “like a son to her”, and Didion replies, “Yes”. Leadbeater goes into another room and cries. They discuss the ending of her novel, Play It as It Lays, and agree letting a suicidal friend die could be a generous act.

She leaves the room and returns with a copy of that book, and inscribes it from “Mommy”. Leadbeater says, “I think she was very maternal to me at a really dangerous point in my life.” Despite her literary reputation as “cool”, in writing style and beyond, that was in stark contrast to how she behaved with her inner circle, he says.

“Her sweetness and warmth and generosity, that was really surprising to me at the beginning,” he says, “because it does not necessarily compute with her public image, which people have become carried away with but she herself cultivated with some degree of intentionality.”

Meanwhile, he is also very aware that he is the “staff”: “there is no tidy word to sum up our relationship”. He says Didion was “never hierarchical” with him, always insisting he stay sitting beside her at the table for starry dinners, but some high-status guests plainly considered it better for him to leave the room. Why did he cry when she said he was like a son?

“Joan’s world offered me an emergency exit, a trapdoor into a wildly free, safe new life.”

“Because I had been so distracted,” he says. He started the job, he confesses honestly, with some ambition to further his literary career. “In fact this entire other play had been going on that I was missing — I was having one of the most important and intimate experiences of my entire life. I had been misreading it. My priorities were wrong. I made that discovery because of love.”

She was defiantly laconic. When Leadbeater confessed his father was in prison Didion said nothing, just rested a hand on his cheek. Frequently, when asked her opinion she replied, “I don’t know.” She hated wasted or inaccurate words.

“She could sit in silence for hours and hours, and it terrified people. It was inherent to her, but then she developed it as part of her journalistic tool kit. She would sit in silence and people would struggle, they’d eventually spill their guts to her, in desperation for advice or approval.”

He describes the day of the Celine shoot. Phoebe Philo, Celine’s creative director at the time, revered Didion. Philo’s “love and affection began to assert themselves more evidently — she was waiting, one felt, for some word of approval to pass from Joan’s lips”, he writes. But instead, Didion sat “almost motionless, silent, tiny as a bird”. It’s a master class from Didion as a woman untraditionally uninterested in making everyone and everything OK.

But sometimes it’s hard to interpret Didion, so precise on the page and yet so gnomic in person. At one dinner party Didion pronounces that Hillary Clinton, then a presidential candidate, was “not totally there”. I press Leadbeater for what she meant and we eventually lapse into the reality that we can’t know. He has a rule, he says, not to speak for her.

Did the fact that she was prepared to remodel her apartment mean she had reached some kind of closure with the deaths of her husband and daughter?

“No. She lived with them for the rest of her life. I’m breaking my rule again, but I don’t care. I think she would say she finds the idea of closure suspicious … she wasn’t trying to achieve closure. She was trying to assemble all of those memories into some new order, a new landscape she could then live in. You can’t reach closure with losing a 50-year marriage or losing a daughter; there is none for those tragedies. And I think she wanted to live with them. She didn’t want to leave them behind.”

It’s clear Leadbeater also can’t leave Didion behind. He doesn’t want to be free. She had a close circle of friends and family, he said. But it’s striking, I point out, that it was Leadbeater at the hospital vigil, Leadbeater collecting her ashes.

“Quintana and John not being there changed the dynamic of how all those things would have worked,” he says. “And I was so willing to do it, I loved her so much and felt she deserved it.”

Their needs, in a way, interlocked. When I say she was lucky to have him he cries and thanks me. Any squeamishness one has about the ethics or taste of a potentially exploitative posthumous “care and tell” book is allayed by Leadbeater’s reverence and his self-aware self-criticism. He tells me that the last year and a half of visiting the apartment after Didion died were spent not living but re-living, “in a suspended state of delusion” that he couldn’t bear to end.

“To continue to return to this apartment, where everything was completely intact … I don’t want to steal the image from The Year of Magical Thinking, but she talks about waiting for John to come back. I was not waiting for Joan to come back. I knew that she was not coming back. It was more like I had been part of a play for ten years and they kept the set perfectly preserved. I could just walk back onto the stage and be there,” Leadbeater says.

“I knew that Joan was gone, but in some ways she was not gone at all, because there was the ashtray and there was the chair she always sat in while we listened to music. And, oh look, the sky is turning grey over St James’s. Clearly there’s a storm coming so we can’t go to the park.”

Helen Rumbelow is a political correspondent for The Times of London