The sixth Beatle

If Pete Best is the fifth Beatle – or is that Stuart Sutcliffe, Brian Epstein or George Martin – then Hunter Davies is surely the sixth. More than Paul McCartney and Ringo Starr themselves, he is still the go-to authority on all things Beatle-related – 44 years after he wrote the only authorised biography of the world's most famous band. 

Davies is one of my very favourite people. Warm, funny, nosy and indiscreet, he is still, at 76, a prolific author and old-school hack. I first met him through a close friend, his nephew, and when Ross lived at his uncle's house one summer in my early 20s, I fell in love with that leafy corner of London, close to Hampstead Heath. Fifteen years later, I live there, as does Davies still. We sometimes meet for lunch, during which he hounds me for Guardian gossip, insists on wine, and supplies me with feature ideas I should be writing.  

This week, nearly half a century after his first, Davies has published his second Beatles book, The John Lennon Letters (on what would have been Lennon's 72nd birthday). Davies turned sleuth, tracking down letters, postcards, scribblings and doodles from all corners of the world. Yet the hardest part was persuading Yoko Ono, keeper of her late husband's estate – and owner of the copyright to all his letters – to let him go ahead with the project. He did it by pointing out that the owners weren't getting any younger. 

Although he has written over 40 books not about the Beatles, Davies will probably be remembered for his Fab Four one. But with good reason: he spent hundreds of hours with them at the height of their fame; was there, in the studio, while they created some of their best-loved songs; was there for the shoot of the Sgt Pepper album cover. But more than that he watched, observed and made sense of them – something they never did themselves. They were too busy being Beatles. 

Doesn't he ever tire of them? Yes and no. As he wrote, by default, the only official biography (they split by the time his exclusivity ended), he could, he wrote in the The Guardian this week, "spend the rest of my life, every day, giving a Beatles talk somewhere around the world. Sad thought". 

But he admits that he was, as a humble hack, privileged to be part of history. Listening to him talk about that time, at the book's launch last night at the British Library, he's as excited as if it happened yesterday. 

He even showed a Super 8 video he shot in 1968 when he was living in Portugal for a year, enjoying the proceeds of his newly published book. Just before Christmas, there was a knock at the door – it was McCartney, with his new girlfriend Linda and her young daughter, Heather. They ended up staying for three weeks. The photograph below was taken during that time. 

Davies is the first to admit he's no match for Beatles experts: he spells things wrong, forgets sequences of events. But, as he pointed out last night, that's what they're for. One fan is writing a taxonomy of the typewriters Lennon used in his life, using Davies' new book as a key reference. I don't think that one will trouble his publisher. 

Hunter Davies portrait: David Woolfall