‘It’s a drag, isn’t it,” Paul McCartney told reporters quizzing him the day after John Lennon’s murder, a soundbite as dispiritingly muted, even callous, as his reaction to his mother’s death when he was 14: “How are we going to get by without her money?” Behind the scenes, Paul was lost and tearful, as well as guilt‑stricken that he and John hadn’t properly reconciled since the Beatles split: “I’m never going to fall out with anybody again.” Still, the enshrinement of John and vilification of Paul had begun. “John Lennon was three-quarters of the Beatles,” Philip Norman told television viewers while promoting his biography, Shout!, a few months later.

The antagonism has abated in recent years, but the John-Paul duality persists. Heavy rocker versus cute populist. Working-class rebel v smug bourgeois clone. Tormented genius v girly sentimentalist. Strawberry Fields Forever v Penny Lane.

Ian Leslie takes on these tired polarities by reframing the story as a volatile bromance: “passionate, tender and tempestuous, full of longing, riven by jealousy”. However much at odds temperamentally, John and Paul were an indivisible twosome, the driving force of the Beatles, with George and Ringo (not much featured here) as add-ons. The emotional ties they shared, not least the early loss of their mothers, weren’t ones they could talk about, so they sang them instead. As Paul put it: “You can tell your guitar things that you can’t tell people.”

To Beatles aficionados, the cast and chronology will be familiar: the Quarrymen, Hamburg, the Cavern, Beatlemania, Abbey Road, the Apple rooftop concert; Brian Epstein, George Martin; Cynthia Lennon, Yoko Ono, Jane Asher, Linda Eastman. But Leslie’s approach is fresh because focused on the double-consciousness (“a duet not a duel”, “a group within the group”) and their “shared ownership of each other’s talent”. He follows them from the teen years when they bunked off classes to strum at each other’s houses (“Paul’s reversed guitar meant that the two of them could act as mirrors for each other”) through jamming sessions in hotel rooms to late-night studio recordings. Their work rate was phenomenal (at the Kaiserkeller they played seven nights a week till two or four in the morning). But so was the intimacy and sense of fun.

“It’s like you and me are lovers,” John once said, to which Paul grunted assent, and even after the band’s breakup they spoke of their relationship as a marriage. John referred to Paul as “an old estranged fiance” and described how getting together with Yoko reminded him how he’d picked Paul “as my partner”. Maybe, John conceded, “it was a marriage that had to end”. Still, “I would do anything for him, and I think he would do anything for me.”

As Leslie sees it, the marriage didn’t end because of musical differences, but because they were spending less time together, and others had intruded. John’s marriage to Cynthia, and Paul’s long romance with Jane Asher (plus countless flings), didn’t seriously threaten it. But Yoko Ono was a force of nature, and with John quiescent in her domineering presence she became a replacement for Paul; John told her he liked her “because you look like a bloke in drag. You’re like a mate.” Linda Eastman was even more of a threat because Paul allowed her well-to-do family to take over Apple’s financial affairs, much to John’s rage and resentment.

After the Beatles broke up, his denunciations of Paul, both in interviews and in music, were ferocious. The Beatles years had been humiliating, he said, with Paul “a pretty face” who made muzak, rather than a true artist; Yesterday, a Beatles song with only one Beatle on it, was the epitome of his soppiness. Paul was hurt and John backtracked, describing him as “my closest friend, except for Yoko”. But John had been hurt too, by Paul’s neglect and bullying assumption of command, and there were further outbursts. Nonetheless, his idiom (“dear one”, “brother”) was still tinged with affection.

For Leslie, the intensity of their relationship is imprinted in their songs. He spends many pages dissecting them musically and is thorough in identifying influences: behind Hey Jude, for example, he discerns Bach, doo-wop, Broadway, Anglican church music and gospel. But it’s what John called the “eyeball-to-eyeball” collaboration with Paul that interests him most: “They liked to put their faces close together and stare, unblinking, until they felt themselves dissolving into each other,” he writes, a tad mystically. Laughter was crucial (the Beatles loved to lark about). So was whistling: “The way we work,” Paul said, “John will whistle at me and I’ll whistle back to him.” Some of their songs were composed within a couple of hours.

Leslie doesn’t shirk the question of how much each contributed to the lyrics and melody of their classics. When John, jealous of Paul’s versatility, claimed to have written half of Eleanor Rigby, it was a wild exaggeration. But the emphasis here is on Lennon-McCartney as a joint enterprise, and the miracle of the songs they wrote together, with their singing voices sometimes indistinguishable and credit to one or the other beside the point: “They were so far inside of each other’s musical minds that it doesn’t matter.”

Reading songs as autobiography is dangerous. Leslie’s previous books have been works of psychology, and he’s an armchair shrink in places here, with Freudian digging to find lines that shed light on the John-Paul relationship. His tone can become overexcited. Please Please Me “a cruise missile carrying a payload of joy”? Twist and Shout “a carnal joyride”? Getting Better a “self-help narrative” in which John acts as “a Greek chorus in the drama of his own maturation”? Well, maybe. Attributing the self-assurance John and Paul displayed on stage to “the arrogance of the damaged” is pushing it, too. Then again, John described Strawberry Fields Forever as “psychoanalysis set to music” and Leslie enjoys the complex identity swaps in the collaboration, such as Paul thinking up the title for John’s book In His Own Write: “There is something delicious about a third-person title being suggested by a second person who co-created the first person’s sensibility.”.

For the most part, Leslie’s book is intelligent, diligently researched fandom. He has read all there is to be read, from the pioneering Hunter Davies to the recent Craig Brown, and has consumed many a film and podcast in between. He’s not afraid of terms such as postmodernist and metatextual. But the tone is chatty and engaging, with the emphasis where it should be, on the songs.

There’ll still be fans wedded to the old binaries. And though Leslie didn’t interview McCartney for his book, he’s not wholly impartial; it was a 10,000-word lockdown essay about Paul that prompted him to go on and write this. Still, his portrait of John’s fragility and self-destructiveness is sympathetic. And his Paul isn’t a winsome poster boy but tough, cynical and prone to cold fury. “‘I realise now we never got to the bottom of each other’s souls,” Paul once said. The dynamic remains mysterious, but this book takes us closer to understanding it.