The inspiration for Patti Smith’s new book could not possibly be any sweeter. Just Kids was written after she made the promise to former boyfriend and dear friend, Robert Mapplethorpe, as he lay dying of Aids in 1989, that she would tell their story.

It’s taken her over 20 years to do, but now Just Kids is a worldwide bestseller. It’s an intimate portrait of a wonderful friendship and is told with wit, irreverence and an irresistible fondness.

Sitting backstage after her gig at Oxford’s Holywell Music Room, having spent the afternoon at an Oxford Literary Festival event so popular it overran by a full hour, Smith explains why it took such a time to complete the book.

“I’ve worked on it intermittently for the last ten years really. Sometimes I would work a lot on it and sometimes I wouldn’t work on it for two years. I had to shape it and sometimes it seemed too ambitious and I had to rein it in and to make sure I absolutely kept the focus on Robert. It was only in the last six months that I was able to really get it down.”

Mapplethorpe and Smith met in 1967, and both would go on to become incredibly successful creative figures. Smith has released ten full albums, including Horses, a record which frequently figures in greatest albums ever polls and a dozen published works of poetry.

Mapplethorpe would become one of the world’s finest photographers, taking iconic snaps of Debbie Harry and Andy Warhol, among others.

Smith ‘remembers’ him onstage in the Holywell, where she is watched by a crowd so reverential that one audience member actually waits to ask her permission to go to the toilet.

Smith speaks with such tangible affection that you’d think Mapplethorpe was in the front row.

Having lost such a dear friend and revisiting a youth that features many other companions now dead, did she enjoy writing the book?

“Sometimes. Sometimes when you’re writing about so many people who are gone, it can be a bit sad.

“But I think I learned a lot writing it and I really think I could tackle anything now. It was instructive as well as giving me the right to say I’d kept my vow to Robert.”

Did the material to write the book come easily, given it’s been over 40 years since they met?

“Well, first of all, I was talking about my childhood and then I met Robert in 1967, so it was a long time ago, yes. But I was helped by the fact that I have a lot of diaries, a lot of journals and lots of letters so I had good primary source material.”

And the process of remembering? Bearing in mind that Smith has always been the kind of artist who constantly produces new material, did she relish looking back?

“Writing the book was very similar to how I work usually. The way I write needs a lot of research, even a poem I’m writing that seems very simple can have seen a lot of research. But I think I would really enjoy researching someone other than myself next time.”

At her gig, things feel as intimate as in her book. Backed only by long-time collaborator Tony Shanahan, she reads aloud from Just Kids, treats Oxford to a couple of her poems and performs a career-spanning selection of songs, including Dancing Barefoot, her ode to late husband Fred Smith, the gentle Grateful and a stirring Paths That Cross.

She ends with a triumphant version of her 1978 hit Because The Night, which has every last person in the room singing to the rafters, causing Smith to grin widely as she exits the stage.

Ask Smith what she’s up to after she finishes her promotional tour and she talks of exhibitions (both of her work and Mapplethorpe’s), a new record and writing another book.

“I’ve got plenty to do” she says, with a smile.

Fantastic, both onstage and in print, and she continues to produce work of astonishing quality.