“I am not a scientist,” says Henry Marsh on the first page of And Finally. “Most neurosurgeons are not neuroscientists – to claim that they all are would be like saying that all plumbers are metallurgists.”

Marsh, who worked as a highly regarded neurosurgeon for more than 40 years, has a penchant for truth-telling, unencumbered by faux modesty. It’s what made his previous books – Do No Harm and Admissions – interrogating a life in medicine, haunted by the “reproachful ghosts” of patients he’d failed, so refreshing and inspiring to read.

This latest autumnal instalment follows in the same vein. Philosophical and scientific conundrums about brain surgery permeate the book: to treat or not to treat patients; how honest to be in giving a prognosis; euthanasia v assisted dying. Along the way the 72-year-old author wrestles with the dilemma of becoming a patient himself.

The memoir’s subtitle and celestial cover design allude to the 1946 Powell and Pressburger film, A Matter of Life and Death. It’s befitting as Marsh reflects on his own mortality after a diagnosis of advanced prostate cancer. He is phlegmatic about his prospects. Sometimes, though, he confesses to paralysing anxiety – a result of his approach towards serious problems that his wife, Kate, calls “therapeutic catastrophising”.

Despite its subject this is not a maudlin book; far from it. Divided into parts like a three-act play, it is often darkly funny, especially in the first act, Denial. Here, Marsh is self-lacerating and also self-forgiving when he reminisces about his medical mistakes. On one occasion he steels himself to admit to a patient that he’d operated on the wrong side of his brain. “Well, I quite understand, Mr Marsh,” the patient answers after a long silence. “I put in fitted kitchens for a living. I once put one in back to front. It’s easily done.”

Marsh is nonetheless fierce on himself throughout the book, as critical as he is of the arrogance of his profession. Now that he’s a patient, he sees clearly how he’s been demoted to an underclass; how some doctors behave as if patients are nothing more than walking pathology; and how they continue to practise medicine under the delusion (once also held by Marsh) that illness only affects patients, not doctors.

Elsewhere, he strikes a sadder personal note, recounting the end of a decades-long friendship with a conscientious Ukrainian neurosurgeon who figured prominently in his earlier memoirs. Working with him in poorly resourced Ukrainian hospitals had left Marsh feeling heroic. But he split from his colleague after discovering he’d been hiding from him a number of cases that had gone terribly wrong, with patients seriously harmed or dying after surgery.

It’s not stated whether Marsh also feels culpable, but certainly he agonises over his professional legacy. That anxiety folds into his nervousness about the future we are bequeathing to our children and grandchildren through inaction over climate change. In one startling passage, he recalls a journey in the Indus delta where he witnessed a catastrophic spectacle: “a flotilla of plastic rubbish … it had neither beginning nor end. It floated past us in complete silence … full of ominous purpose”.

The retired neurologist, who in medical parlance has “hung up his gloves”, has composed a richly discursive book. He charts his ambivalence about undergoing radiotherapy for his cancer, and is especially passionate when advancing the case for assisted dying. He’s scornful of the “dishonest fudge” around the issue that sees doctors accepting the unofficial practice of prescribing large doses of opiate painkillers, as a form of “terminal sedation”.

During Covid, and the cult of death it seemed to spawn, Marsh was animated by the fear his time could run out before he finished making a doll’s house for his granddaughters. Its construction – a mournful metaphor for innocence that a future governed by global warming will deny his grandchildren – is also an act of defiance.

And Finally sounds increasingly ominous about his prostate cancer as the memoir works its way towards a resolution; Marsh is plain-speaking without being dispassionate, almost as if volunteering his own medical history as a case study. Indeed his book reminds me of the mantra – focused on operations – that I first heard at medical school, for doctors embarking on a career in surgery: “see one; do one; teach one”. Henry Marsh may have retired from medicine but let’s hope he keeps producing books as good as this one, which enthral as well as teach.

• And Finally: Matters of Life and Death by Henry Marsh is published by Jonathan Cape (£16.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply