The first two shots went wide. But the third, fired at point-blank range, got him in the chest. The bullet went through his stomach, his liver, his spleen, and both his lungs. In the half hour it took the ambulance to arrive, he bled slowly to death. By the time the medics had him in ER he had no pulse, no blood pressure, and his skin was the same colour as his Blue Jackie (1964). At one point, as the wait for a doctor went on, Andy Warhol was actually pronounced dead.

Only thanks to brilliant surgery (described in Hannibal Lecterish detail in Blake Gopnik’s Warhol — “an oesophagus severed from the stomach, so that food and gastric acid were spilling out from below” anyone?) did Warhol survive. For the rest of his life he would proudly show off what Gopnik calls the “network of Frankenstein scars” the operation had left him with. This isn’t writerly melodrama: as the Richard Avedon photograph on display at Tate Modern’s (with luck only temporarily closed) Warhol retrospective shows, the surgeons didn’t make a great job of stitching him up. They didn’t think it mattered because he hadn’t long to live.

Mortality was Andy Warhol’s great subject. He was deadly serious about it

In the event, he had another 19 years, only dying after gallbladder surgery in early 1987. Still, Gopnik is convinced that 3 June, 1968 — the day Valerie Solanas, a crazed feminist playwright desperate for sponsorship, gunned her idol down —was a hinge-point in Warhol’s life. Although the operation Warhol underwent was as successful as it could have been, his health never really recovered from the trauma of that day.

Nor did his work. As the Tate show proves the art Warhol produced in the five years or so that began in the summer of 1962 still pulses and reverberates. After that, though, it took a long, vertiginous tumble downhill. The bulk of the work he produced from 1968 through to his death is tired and tiresome — flabby footnotes to a once urgent talent. (Although the show is closed, you can look at the work in the Tate’s sumptuous catalogue. Precisely because Warhol utilised mass-production processes into art, his work reproduces with facile grace. Indeed, as his first dealer, Leo Castelli, once half-joshed the critic Harold Rosenberg about a show of Warhol’s silkscreened flowers, there was “no need to see it. The pictures are the same as the announcement I sent you.”)

Warhol’s talent grew out of his piety. As Gopnik reminds us, Warhol was a churchgoer throughout his life (though given the short attention span his work celebrated, it’s little surprise that he’d complain if he thought mass went on too long). Indeed, according to Robert Shore in his brief but instructive Andy Warhol, the family considered young Andy a potential priest.

Nothing came of it, of course, but something of that religious spirit seeped into Warhol’s best works. Six decades on it’s clear that the silkscreens of Elvis and Marilyn and Marlon are more than just paeans to the fame Warhol unabashedly desired. They’re votive images, hymns to the sacrifices celebrity entails. Elvis I and II (1963-4) gives us not the cool Elvis of mid-Fifties rock and roll, but the castrated — crucified? — Elvis of those lousy Sixties Hollywood movies. The vast area of blank canvas in Double Marlon (1966) speaks to Brando’s maunderings on the hollowness of stardom. Not for nothing did Warhol first paint Monroe immediately after her death.

Mortality was Warhol’s great subject. He was deadly serious about it. If there’s something voyeuristic about his Black and White Disaster series (the Tate has one involving a car crash in which five people died, as well as Warhol’s ghostly reproduction of a photograph of a woman who’s thrown herself off a tall building), then that only adds to their unsettling air. Far from being glib tabloid cash-ins — charades of feeling — Warhol’s studies of death are his most potent and personal work.

But how quickly the power drains from his images after the Solanas attack! Facetious to suggest it might have been better had Warhol died from his wounds — except that he himself mooted the idea. “I always wished I had died,” he admitted in his 1985 photo diary America, “and I still wish that, because I could have gotten the whole thing over with.” Certainly it would have spared the rest of us. What the great bulk of his post-’68 work shows is that a Warhol unafraid of death is a Warhol whose talent has died.

Alas, you won’t come across anything as damning in Warhol. For an art critic, Gopnik is strangely uncritical. To read him is to be told that Warhol was one of the greatest artists who ever lived. “It’s looking more and more,” he claims, “like Warhol has overtaken Picasso as the most important and influential artist of the twentieth century.” So high has he flown that’s he’s now up there “on the top peak of Parnassus, beside Michelangelo and Rembrandt and their fellow geniuses”.

For Gopnik, Warhol never put a foot wrong, never made a mistake, never had a bad idea, never made a bad picture. The only times Gopnik admits to doubts about a Warhol work is when Warhol himself fears it isn’t up to scratch. Otherwise, he was incapable of failure. “Warhol didn’t think outside the box,” fawns Gopnik. “He thought outside any artistic universe whose laws would allow boxes to exist.” This is nonsense, and not just because Warhol actually made his name with a box (the Brillo carton replicas he made by silk-screening blocks of wood). The blurb calls the book “definitive”, but deifying would be nearer the mark.

Still, if you want to know what Warhol was doing on pretty much any day of his life, Gopnik is your man. His book is the product of years spent beavering away in the Warhol Museum in Andy’s hometown of Pittsburgh. More than 100,000 documents — letters, diaries, tickets, tax-returns, billets doux — are stored there, and one feels confident that Gopnik has read them all. Certainly he’s got the goods when it comes to money. He knows better than Warhol did what was coming in and what going out. On several occasions he points out that Warhol’s income in his early years as a fine artist was dwarfed by what he’d earned as one of Manhattan’s go-to commercial illustrators in the ’50s and early ’60s.

Gopnik is good, too, on Warhol’s sex life. Because, yes, there really was one. In public, Warhol came on as a frosty hermaphrodite. Sex, he once counselled a potential partner, “is messy and distasteful”. Yet it turns out he was rarely without a boyfriend, loved performing oral sex, and was once treated for a case of the clap. Given its obsequious tone, Gopnik’s was never going to be a warts-and-all biography. Nevertheless, the fact that Warhol once suffered from anal warts is duly noted.

But just as his art became desiccated with the years, so did Warhol’s sexuality. Thanks to his rough, patchy skin — as a child Warhol had suffered from Sydenham’s chorea (then known as St Vitus’s Dance) — and his early-onset baldness — by the time he was 30, Warhol was wearing a wig (three of them are on display at the Tate: they look like ash-trays made out of sporrans) — Warhol had always been self-conscious about his appearance. But after the shooting, with the havoc it had wrought written across his torso, he largely gave up on sex. What he liked most in the last 20 years of his life was watching — and photographing — pretty young boys playing with one another. The alienated mood of Warhol’s best work, that sense that the images close us off from what they’re inviting us to gaze upon, grew out of this sense of distance.

Gopnik is less convincing on Warhol’s politics. He claims that Warhol was a feminist who believed “women are the world’s major artists” and that he “made more space for women . . . than did the vast majority of male artists of his day”. Maybe so, but it’s also true that he was never seen out and about without at least one piece of semi-clad eye-candy — Edie Sedgwick, “Baby Jane” Holzer, Nico — in tow.

Mind you, Gopnik also believes that because Warhol voted for Henry Wallace in the 1948 presidential election, for Jimmy Carter in 1976, and in 1972 presented the Democrats with a campaign poster depicting Richard Nixon as the Wicked Witch of the West, he was some kind of left-wing activist whose work has “anti-capitalist leanings”. This is fantasy. Warhol’s politics, such as they were, went no deeper than consumerist liberation.

“You can be watching TV and see Coca-Cola,” he once said, “and you know that the President drinks Coke, Liz Taylor drinks Coke, and just think, you can drink Coke, too. A Coke is a Coke and no amount of money can get you a better Coke than the one the bum on the corner is drinking.” Yes indeed — though it ought to be pointed out that a quarter of a century after Warhol’s death one of his Coke bottle silkscreens went for $35 million at auction.

And yet — despite Warhol’s candour about his being on the make, there remains something pantheistic in his depiction of the world and its objects. Those Coke bottles, those soup cans, those Brillo boxes — they’re done with such worshipful delicacy and precision, such fondness for the overlooked and quotidian, that they can feel like the last word in Emersonian transcendentalism.

John Updike once said he wanted his work “to give the mundane its beautiful due”. Warhol gave the world of mass-production its beautiful due. Blake Gopnik isn’t a good enough stylist to return the favour. But he gives Andy Warhol all the due he can.