You push through furs to hide at the back of the wardrobe. Suddenly, fur gives way to twigs, floorboards to snow. A lamppost glimmers ahead...

Lucy’s entry into Narnia is an exhilarating transition from the mundane to the mysterious, out of confinement and into endless unexplored vistas. This passage transformed me, age 6, from an idle reader of comics into a glutton for books.

Other passages to other worlds have followed. Alan Garner guided me into the hill of his native Alderley Edge, where a king slumbered awaiting Britain’s greatest need. J.R.R. Tolkien led me from the Shire onto the road to Mordor. In her The Dark Is Rising sequence, Susan Cooper opened a window between the modern Home Counties where I grew up and a version of our world which seemed more meaningful, defended against the tyrannical Dark by a circle of wizardly Old Ones. In adulthood,Philip Pullman’s His Dark Materials took me from a steampunk Oxford into a whole multiverse of interconnecting worlds.

Now in our own Oxford, Pullman has opened an exhibition at the Bodleian Library which celebrates these authors and the tradition they inherit from medieval times and earlier. Magical BooksFrom the Middle Ages to Middle-earth recreates a library within a library, and includes artworks and manuscripts of Tolkien, C.S. Lewis and Garner which reside at the Bodleian. In a corner dressed as a Narnian snowscape visitors may listen to recordings from their works. A tagline comes from Lyra’s Oxford, on view in Pullman’s manuscript: “Oxford, where the real and the unreal jostle in the streets, where windows open into other worlds.”

Tolkien built Middle-earth in Oxford, beginning as an undergraduate when the First World War shattered the façade of ordinary life. In he stirred Norse myth, Finnish folklore, William Morris’s romances, Peter Pan. It was not mere escapism. Deathless Elves embodied something akin to Keats’s equation of truth and beauty – ideals abandoned in the insane industrial war. After service on the Somme, Tolkien made Middle-earth an arena of perpetual conflict between the Elves and the kind of tyrannical materialism which had ploughed Europe’s soil and sons into a morass of putrefaction.

As Oxford dons in the mid-Twenties, Tolkien and Lewis recognised each other as kindred spirits — both in love with mythic northernness, medievalism and “the horns of Elfland faintly blowing”. Without Lewis’s urging, Tolkien might have left The Hobbit unfinished and never started The Lord of the Rings. Without Tolkien’s defence of human myth-making as evidence of a divine Creator, Lewis might never have found the Christianity which shaped Narnia. Their two worlds appear in the exhibition in a case of maps, a vital fantasy ingredient: Middle-earth illuminated by Pauline Baynes, Narnia sketched by Lewis.

The Oxford English syllabus reflected the pair’s uncompromising stance that medieval trumped modern. Many students hated this, including in the mid-Sixties Pullman himself, who loathes Tolkien and Lewis. He calls Narnia “blatantly racist” and“monumentally disparaging of girls and women”. It’s fighting talk from an author who has drawn from Milton and Blake to reshape the tradition.

But Cooper, who took the same course ten years earlier, says: “Their insistence that the basic English syllabus should stop at 1832 certainly shaped my imagination, because of the resultant stress on early and medieval literature – Beowulf, Gawain,Spenser, Milton, Malory, above all Shakespeare.”

At the exhibition is the latest addition to Britain’s oldest continuous literary tradition: a page in Tolkien’s finest calligraphy from his poem The Fall of Arthur, published by HarperCollins. It’s even more apt now than when the exhibition was planned, as new evidence suggests an Arthurian ur-text, Geoffrey of Monmouth’s History of the Kings of Britain, was written in Oxford. Two pictures at the exhibition show how Arthur has answered the needs of many hours. In a 15th-century illumination, he watches knights joust with the finesse of dancers. In a 1917 Arthur Rackham illustration, Mordred and the King blindly hack and stab at each other in a Passchendaele of mud and corpses. Cooper’s Old Ones include Merlin in modern guise.

Garner, her exact contemporary, read Classics at Magdalen College, where Lewis was a don. No fan of Narnia – “I’m more than with Philip Pullman on that one” – Garner admires Tolkien’s scholarship and enjoyed The Hobbit but wearied of The Lord of the Rings. “Too long: I’m a minimalist,” he says. His own debut, The Weirdstone of Brisingamen, was published by William Collins in 1960 to capitalise on the success of Tolkien’s epic.

Garner brought other influences to his work, including Aeschylus and – discovered at 7 – the Ramayana. “Monkey gods and mountain shifters and – oh boy! that was for me!” The children’s escape through a warren of tunnels is the most original aspect of The Weirdstone. “That’s because I didn’t make any of it up,” says Garner. He was exploring the tunnels with friends in summer 1956. “Cramped ten minutes in the Devil’s Grave,” his diary records. “Second descent of the West Mine. We took Scamp but he would not cross the plank shaft.” “Cascades and floods; a new gallery with a fearsome shaft.” Later that year, Garner dropped out of Oxford and returned to Cheshire to finish his novel.

He visited when writing the sequel, The Moon of Gomrath, to examine magical scrolls at the Bodleian – in a gated alcove alongside “cloned Oxford ladies in late middle age, wearing sensible brogues, lisle stockings, tweeds and flat hats secured with long silver pins”. The alcove was for pornography, but also because the Fraudulent Mediums Act of 1951, which replaced the Witchcraft Act of 1735, was still a matter for concern. He told the librarian he had changed some spellings in The Weirdstone incantations just in case. “And he said that I was wise to do so . . . not because he believed in the words’ efficacy but because others certainly did. I still wonder about those ladies.” The books Garner consulted are on display in the new exhibition, alongside the table used by John Dee to converse with angels and the Ripley Rolls which instruct on making the Philosopher’s Stone.

Garner has loaned a plate decorated with an ambiguously floral design – the inspiration for The Owl Service, his Carnegie-winning 1967 novel in which all resemblance to Tolkien ceased. Curators Judith Priestman and Sarah Wheale have traced its patterns to make predatory owls, just as teenage Alison does under the possession of an unquiet spirit from Welsh legend. Cooper has loaned six quartered circles made by her first husband in celebration of the magical signs of wood, bronze, iron, water, fire and stone in her books. A case about divination includes the alethiometer, the truth-telling device from His Dark Materials, commissioned by Pullman. Dr Priestman says, “You feel all these things should work. They do feel very powerful.”

Originally, she adds, the exhibition title was to be The Dark Is Rising “to encapsulate what all of these books are doing: the Dark is rising and somebody comes to stand against it with the Light”. Cooper, who witnessed the midnight sky lit as if by a sunrise during the London Blitz, says: “The subconscious effect of a wartime childhood was paramount.” The bogeyman in her nightmares was a German paratrooper.

The only previous Bodleian exhibition to cover any of this ground was one in 1992 on Tolkien’s life and academic work. Blockbuster movies have since brought Middle-earth and Narnia to an even wider audience. Lewis gets a plaque at Poet’s Corner this November, the 50th anniversary of his death, and is the subject of a high-profile new biography by Alister McGrath. Garner recently published Boneland, an adult sequel to his first novels, and there are Folio Society editions of The Dark Is Rising. Children whose imaginations were shaped by Garner and Cooper are now authors and opinion formers. The time is ripe.

Magical Books has a more fundamental message for the era of movies and videogames. The word, bearing the idea of things to hand or even out of sight, is still a magical thing. Fantasy carries the power of words one step further: to things that have never been seen, or could never even be. Cinema can stand in for the imagination: a CGI glut with an aftertaste of corn syrup. But reading engages our own imaginations instantly and inexhaustibly. The images may remain with us for ever, so simply walking up a hill may return us to hills that never were, where the next cleft may conceal a dragon. No wonder that Tolkien’s are among the most thumbed volumes in the library used by the prisoners at Guantánamo Bay.

The Bodleian Library’s Magical Books – From the Middle Ages to Middle-earth runs until October 27. A book edited by Carolyne Larrington and Diane Purkiss accompanies the exhibition. John Garth is the author of Tolkien and the Great War.

This article first appeared in The Times. It is reproduced with kind permission. © John Garth