A few months after the death of the neurologist and writer Oliver Sacks in 2015, I visited his office in midtown Manhattan to pass on my condolences to his friend and longtime editor Kate Edgar. Shelves were stacked with editions of every translation of Sacks’s books; piled on every surface were reams of yellow paper covered in Sacks’s barely legible scrawl. The subjects were as eclectic as his enthusiasms: two towering piles were marked “Dreams” and “Memory”, and another was marked “Borges”. There was more than enough material for at least two more books of essays, Edgar told me (The River of Consciousness would be published in 2017, and Everything in Its Place in 2019), and then, gesturing to some of the piles around her, intimated the scale of another task: sorting through 70 large boxes of correspondence.

From the riches of those boxes, Edgar has now distilled 700 pages of absorbing, illuminating, moving and entertaining letters for publication. They range from August 1960 (when Sacks first moved from England to North America), through to a few days before his death in New York, from melanoma, 55 years later.

Sacks qualified in medicine in England in the 1950s and, after his internship, began training as a neurologist in California in 1961. The US offered “enormous opportunities, not merely professionally, but for leading a full and fascinating life in many other ways”, he wrote home. He came from a family of medics: his father, Samuel Sacks, practised as a London GP well into his 90s, while his mother, Muriel Elsie Landau, was a distinguished gynaecologist. She looms large throughout this collection: “My mother was not only the first Jewish woman (and almost the first woman) to be elected a fellow of the Royal College of Surgeons,” Sacks later wrote, “[but] one of the very few Jewish consultants in the whole of England.” One letter reveals how she would occasionally bring home “deformed foetuses” from hospital and ask the 10-year-old Oliver to dissect them. He had a traumatic childhood: he was evacuated to a sadistic residential school during the war, and his letters often return to the horror of that abandonment. His memoir On the Move (2015) describes how his mother abandoned him twice over when, at 18, he admitted his homosexuality. “You are an abomination!” Sacks remembered her saying. “I wish you had never been born.”

Sacks wrote of Kate Edgar as his ‘mother figure’, adding that he did his best work when she was in the next room

But the loving early letters Sacks wrote to both parents are among the jewels of this collection, detailing his anxieties about medicine (citing a temporary “aversion to patients, sickness, hospitals and particularly doctors”), his weightlifting escapades (he was the joint Californian squat champion), and his accounts of motorbiking through the desert, high on amphetamines, and using a stethoscope to siphon stolen petrol.

Even in his 20s, Sacks wrote in a learned, stylish and energetic prose. But it’s after his move to New York in 1965 that the compassionate physician of later fame appears. He began with neuropathology (but confessed “poring down a microscope drives me nuts, and I need some return to clinical work”), then took on a job in a headache clinic – material from which would become his first book, Migraine (1970). Then, as a resident neurologist at Beth Abraham, a hospital for chronic neurological disease, he found his vocation, as devoted physician to patients with a severe form of Parkinson’s disease (encephalitis lethargica), all of whom had been institutionalised since the “sleepy sickness” epidemic of the 1920s. A new medication, L-dopa, was just becoming available, and in his groundbreaking Awakenings (1973) Sacks charted its spectacular effects. The book later inspired a play by Harold Pinter and a Hollywood movie starring Robert De Niro and Robin Williams. In the early 80s, a young editor called Kate Edgar was assigned to edit a book of his neurological case histories that would later become The Man Who Mistook His Wife for a Hat (1985), the title chosen by his then editor at the London Review of Books, Mary-Kay Wilmers.

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A letter from Sacks to Edgar asking her to stay on working for him is among the most touching in the book: he wonders “whether we might make some sort of private arrangement between us, my retaining you as – I can think of no term general enough! – but as some sort of literary/intellectual factotum, inciting, organising, typing, editing… odd pieces, case histories, essays, lectures, etc”. It comes two-thirds of the way through Letters, when Sacks is 53; Hat (as he called it) propelled him to the bestseller lists and international fame.

Sacks wrote of Edgar as his “mother figure”, adding that he did his best work when she was in the next room. His many subsequent books – Seeing Voices, Uncle Tungsten, Musicophilia, Hallucinations, among others – are testament to a successful partnership. By the end of his life, his correspondents included his many fans around the world (in my loft at home I have a kind response he sent me in 1997), as well as poets, artists and scientists. The prose shifts gradually in tone as he was called upon to take up the role of public intellectual, but his restless curiosity, breadth of interests, and the energy of his enthusiasm never wanes, even when responding to his critics. His reply to a psychiatrist, Orrin Palmer, who in 1988 wrote to complain of Sacks’s characterisation of Tourette syndrome, reads: “I have a profound respect for Affliction, and the Afflicted – this is at the heart of my physicianhood and personhood… While I fully respect the medical model – and my own writings (Migraine etc) are examples of it – I think that one must go BEYOND it.” Sacks showed generations of doctors (and patients) how medicine is just the starting point for an exploration of the possibilities of being human. With these letters, his legacy as an extraordinary writer, humanitarian and physician is secured.

Gavin Francis is a GP in Edinburgh. His book The Bridge Between Worlds: A Brief History of Connection is out now

Letters by Oliver Sacks, edited by Kate Edgar, is published by Picador (£30). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply