Philip Pullman has often said that "it takes Once upon a time' to reach the heart". And on March 31, he will headline the Oxford Literary Festival, to unveil the latest episode in the His Dark Materials universe - his new book, Once Upon a Time in the North.

It tells of the first meeting between the Texan aëronaut Lee Scoresby and Iorek Byrnison, the armoured bear.

"I'd always wanted to tell this particular story. Because when we see them in Northern Lights for the first time (as Lyra does) they're old friends, they've known each other for donkeys years."

"In fact, I stole the idea of this story from The Magnificent Seven. There's a moment there when the two main characters, Yul Brynner and Steve McQueen, who've never met before, join forces in this sort of laconic way to . . . get something done."

And so Once Upon a Time in the North begins, with "a lean young man with a large hat, a laconic disposition, and a thin moustache" crash-landing his balloon in the Arctic harbour town of Novy Odense.

Scoresby is a drinking, gambling, smoking, brawling flirt, and a mercenary. "Well, this is a story about two grown-ups. I hope Lee's an attractive protagonist; but no, he's not a Boy Scout." However, he always does the right thing, and I don't recall him ever being paid.

"That's right, and it wasn't until I was well into this story that I realised it was about honour. Lee follows a pattern in this book that he's going to follow throughout his life. He's always going to do the honourable thing, but reluctantly, and knowing where the money is." And that it's nowhere near him.

Integral to any hero's character are his noble steed (or "battered cargo balloon") and his weapon. Here we see Scoresby being given his trademark Winchester rifle. Along with the balloon, it foreshadows much. "I was pleased to make the link. I am assuming that most people who read this book will have read His Dark Materials and will remember Lee's death, and the fact that he's holding this same rifle will be a little connection for them."

Pullman maintains that the characters in his head have not changed one bit since he saw them on the big screen (in The Golden Compass), but "Sam Elliot, who played Lee Scoresby in the film, was just fantastic. Couldn't have been better casting. It's the role he's played all his life! Sort of laconic and witty and very laid back. But, of course, in the film he's middle-aged and here he's young."

Very young, in fact. He's 24.

"It would've been great to have a picture of the young Sam Elliot in Once Upon a Time in the North, but we couldn't find one."

The new book follows the format of Lyra's Oxford, including a board-game/map and other "ephemera", as well as a new set of illustrations by John Lawrence. ("A wonderful illustrator," says Pullman, "a real proponent of the English style of book illustration and wood-engraving.") It is a free-standing episode in the universe of His Dark Materials, taking place long before Lyra Belacqua is born.

But the author says: "I thought I ought to mention her at the end. Let's say we see a little glimpse of Lyra's future."

It is full of typically understated humour - the reason Scoresby can't land the balloon is because he owns only the first half of The Elements of Aerial Navigation - much of which is provided by Scoresby's dæmon, Hester, a sort of truculent older sister crossed with a tough-love boarding school matron.

"She's a scrawny old thing, and she's very laconic. She's modelled on my aunt, actually!"

You won't find Novy Odense on any (other) map; but it represents "any part of the world where democracy is tenuous and the rule of law is weak and greed is rampant".

"The forces that are eating away at democracy" cast long shadows over Scoresby's more uncompromising attitude to life, echoing Pullman's frustration with much in real-world politics. Do we occasionally need someone like Scoresby to get something done?

"Yeah, but of course they can only flourish - or exist at all really - in places that are a bit of a frontier. You can't live the Magnificent Seven sort of life in Botley!"

But you can in Pullman's universe. And when the good fight needs to be fought, Lee Scoresby doesn't stand around.

"No, well, he's the character who would be played, whenever he lived, by Sam Elliot. He's the honourable cowboy, the Western hero."

  • Once Upon A Time In The North is published by David Fickling at £9.99. Philip Pullman will be speaking at the Oxford Literary Festival on Monday, March 31