Why has no one ever written a book like this before? It simply tells the stories, with great tenderness, insight and self-doubt, of a phenomenal neurosurgeon who has been at the height of his specialism for decades and now has chosen, with retirement looming, to write an honest book. Why haven't more surgeons written books, especially of this prosaic beauty? Of blood and doubts, mistakes, decisions: were they all so unable to descend into the mire of Grub Street, unless it was with black or, worse, "wry" humour?

Well, thank God for Henry Marsh. His speciality is drilling into people's heads and sucking out or cauterising various problem globules, usually life-threatening. Those are the bald basics, but they disguise a multitude of traumas, not least those of a very human surgeon. He writes with near-existential subtlety about the very fact of operating within a brain, supposed repository of the soul and with myriad capacities for emotion, memory, belief, speech and, maybe, soul: but also, mainly, jelly and blood. He has been 4mm away, often, even with microtelescopes, from catastrophe.

"As I become more and more experienced, it seems that luck becomes ever more important." Not the most copper-bottomed reassurance you could wish from the man who's going to plough your brain, but honest. And he has removed so many problems, with filigreed sure-handed finesse: there was a 15-hour operation once, but it had to be attempted. "The skull is a sealed box and there is only a limited amount of space in the head."

He's been to Kiev, given selfless time there to fledgling neurosurgeons who might as well have been working with flints and candlelight, and saved many lives there too.

But he doesn't flinch from admitting disasters. His chosen word is "catastrophic". It applies to bleeding within that sealed nut of the skull, as in "Once I had sawn open the woman's skull and opened the meninges, I found to my horror that her brain was obscured by a film of dark red blood that shouldn't have been there." He has "wrecked" patients, he woefully admits; patients left half-frozen, half-crippled, dead. But there was no option. Or was there? One of the finest admissions to emerge in this phenomenal book is that of every surgeon's dilemma, which is the inability to play God: but instead to have to decide, after nights of soul-searching, whether it's worth it. All moral oversimplifications steal away like morning mist.

Throughout, there runs a caustic commentary on the current target-setting woes of the NHS. Patients being shunted, at 3am, not between wards but between hospitals, sometimes 150 miles apart. Not the Ukraine, quite, but the idiocies could give it a run for its money.

"I have lost count of the number of different passwords I now need to get my work done every day."

He tells, briefly in the last chapter, the story of having to race up various flights of stairs, repeatedly, to ascertain a password for a ruinously expensive NHS-wide computer system, just the latest in a succession. "Try Mr Johnston's," he's told. "That usually works. He hates computers." Forty-five months have passed since the introduction of the latest doomed system. The password is "Fuck Off 45".

Marsh tries it back in his office, in various upper/lower case and space-optional guises. He is sitting before a policeman who has had sudden serious epilepsy attacks, and his ageing parents, and waiting to get into the system to find the relevant x-ray, which will probably save the man's life. He has to run again, two flights up, to double-check the password. Two months have elapsed. Turns out it is now "Fuck Off 47."

Apparently Mr Marsh's decision to retire has been hastened by the threat of disciplinary action, at the hands of an NHS manager, for wearing a wristwatch on his rounds. There is no evidence of the risk of infection being infinitesimally increased by the wearing of such. What a bloody loss. And what a bloody, splendid book: commas optional.