There is a box file marked ‘Jenny Diski (Simmonds) School and other early’ next to my desk. It’s been sitting here for a couple of years, and, just as in childhood when I would flick through the contents to fill an hour on an empty Sunday, I’ve left it largely untouched. On those Sundays the thing that drew my attention was an onion-thin letter, worn with rereading, in which Doris Lessing offers my mum a room, and an alternative existence.

When teenage Jenny, then Jennifer Jane Simmonds, opened it as she lay in her mental hospital bed recovering from an attempted suicide, I imagine she tensed even more than usual: ‘What I am offering you, to be quite clear about it: is a home here, with a room of your own.’

‘Fuck,’ she must have thought.

Jennifer’s reply is more composed than that, sweet even: ‘Your letter came as a complete surprise to me, and I was quite tearful when I read it – I thought people stopped doing wonderful things like that in Cinderella’s day.’

Doris had never met the 14-year-old she invited to live with her in Camden Town. The girl who turned up on her doorstep a week later was far less accommodating, and far more sensitive, than the girl Doris had imagined. Her son, Peter, who told Doris about his school friend’s situation, prompting the offer, may have given his mother some sense of her, but Doris couldn’t have expected a child with the same forceful temperament, and the same deep, gnawing desire to write, that Doris herself had always had.

The box is interesting because the forceful temperament that Jenny was known for, in person and in her writing, isn’t really in there. An incomplete diary from Jennifer’s school days warns ‘beware!’ before pages of slushy poems, very short stories, and brown smudges of blood. There’s a manuscript that looks like the start of a novel, probably written when she lived with Doris. It’s the detailed story of a girl called Rosalind’s parents’ screaming rows, her far too appealing father and her hapless mother. There are letters between Jennifer and Doris, a dozen from that appealing father, personal records of her time in ‘looney bins’, and a bit more writing, but not much. All of it is Jenny Diskish, but caught in wisps of a significantly less confident Jennifer Jane.

The silent ‘I can’t think, I can’t write’ monologue, which Jenny thought and wrote about brilliantly in her later work, overwhelmed her until her mid-thirties. At 37, death became the only alternative to a life of not thinking and not writing. Luckily for both of us, that was also the age that she sat down in our living-room in West Hampstead and wrote her first novel, Nothing Natural, well enough to avoid it. ‘It’s the knowing you won’t get help however urgently you want it that ratchets the feeling up into madness, a spiral that runs out of control,’ she once wrote in an LRB review.Becoming a writer was the only help that really worked. Being a writer, for Jenny, meant being a paid-up professional and, most important, existing in the only way she could tolerate.

A fraud is being perpetrated: writing is not work, it’s doing nothing. It’s not a fraud: doing nothing is what I have to do to live. Or doing writing is what I have to do to do nothing; doing nothing is what I have to do to write. Or: writing is what I have to do to be my melancholy self. And be alone.

She wrote that in her first piece for the LRB, in 1992. It opens her new book of essays from the paper, Why Didn’t You Just Do What You Were Told?, selected by Mary-Kay Wilmers. Jennifer and her box of childhood doubts are in there, but like her other subjects – Keith Richards, Piers Morgan, Richard Branson, Princess Diana, Jesus (all rule-breakers of sorts) – they have been observed and remade under Jenny Diski’s cool gaze. In each piece, she exposes every hint of bullshit, though not without an acknowledgment of and respect for difference. ‘Different strokes for different folks’ was a well-worn household phrase in my childhood. Jenny’s stroke, however, could cut to the bone.

Over the years, I noticed three things made life easier. The first was becoming an ‘author’, the second was Prozac, and the third was writing for the London Review of Books. (Between 1992 and 2016 she wrote 144 pieces for the paper and 65 for the blog.) The last piece in the new collection, ‘A Diagnosis’, was published in the paper in 2014. ‘I’m a writer. I’ve got cancer. Am I going to write about it? How am I not? … Writing is what I do and now cancer is what I do, too.’ She wrote 17 more pieces, revealing the various ways her past and present self lived at the edge of life. They were collected as a book, In Gratitude.

Her final years were like anyone’s final years with cancer: shocking, tragic. The writing helped more than the drugs. It helped her to live in the only way she could manage and, in the end, it helped her to die in the only way she could manage, too.