Philip Pullman arrives at the Ashmolean Museum in Oxford wearing a jaunty beret and donnish cord trousers. At 64, he looks a little like fellow-author William Boyd, or a neat, unstained version of John Bayley. We are here to discuss World Book Night, which takes place tomorrow - the first event of its kind and the brainchild of Canongate publisher Jamie Byng - when one million books will be given away by and to members of the public in a great and almost-spontaneous celebration of the written word.

Twenty-five titles will flood the streets, including Pullman's Northern Lights, the first in his multi-million-selling His Dark Materials trilogy. He is, of course, delighted by the scheme. Pullman is courteous and obliging, mild-seeming; essentially a reluctant sort of soapbox activist - yet occasionally, when he talks about the axe that hovers over up to 800 libraries, he visibly shudders with anger.

Over the past few months Pullman has become the unofficial spokesman for the campaign to save the hundreds of libraries across England and Wales threatened by public spending cuts. An impassioned speech he gave at Oxford Town Hall, in which he mocked the Big Society, has recently "gone viral" online. More than 20,000 people have downloaded it, while the novelist Joanne Harris and the actor Samuel West both immediately tweeted their bravos.

"The decision to close so many libraries so quickly by so many councils must be ideological," he spits. "It's an act of gross stupidity carried out by people who don't read very much and don't understand the value of books - and the evidence for that is that some councils have decided not to close any libraries at all - but it also comes from central government, from high up. From people who have gone about their business with an ideological fervour which even eclipses Thatcher.
And I'm particularly angered by the Lib-Dems for supporting it."

Pullman is not unfamiliar with the tumult of public debate. In the US, His Dark Materials was condemned by a particular strain of fundamentalist Christian as anti-religious and atheistic (which it is) and therefore a menace to the nation's children. The film adaptation of the first novel - released as The Golden Compass, starring Nicole Kidman and Daniel Craig - was boycotted by The Catholic League and failed to make money in the US. Last year, Pullman went one better and published his re-imagining of The New Testament minus the supernatural miracles in The Good Man Jesus and the Scoundrel Christ, a compelling expression of his belief in stories, and not a higher power, as the font of a collective morality. He describes his enemy as "zealotry" - whether religious or political or ideological.

Plus the Lib-Dems. He really hates them. Pullman voted Lib-Dem in Oxford West because he thought the then sitting MP Evan Harris had done a decent job, "but it didn't enter my mind for a single second that his party would then support the Tories and that my vote would go to a coalition government bent on such destruction".

Provocatively, he states that David Cameron, George Osborne and Nick Clegg have "probably never used a public library" in their lives.

"They come from rich backgrounds and live in places where there are nice bookshops and they just buy books when they want them."

Inevitably there has been a backlash. Some centre-Right commentators have pointed out, in explicit reply to Pullman, that library use by adults has been falling (almost 40 per cent of adults have used a library in the past year, down from 48 per cent four years ago, though among primary school children it has stayed constant at 78 per cent) and that they are least used as public facilities by "deprived" communities - the very people whom Pullman identifies as in most need of them.

"And that's a reason for taking them away?" he counters, shaking his head vigorously. "It's a reason for keeping them, and making them more available and helping people to use them, and encouraging them." He talks of the single mum with little children who perhaps can't afford to bus her brood out to the one remaining library in town. "It's why libraries on inner- city estates must be saved at all costs." Pullman can't help but sound patronising when he says young mothers need to be taught not only how to sing and read to infants but "how to hold their baby" while they do it. But he means it quite sincerely.

Pullman learned to read at the age of five, while closeted away on a boat to South Africa. His father was an RAF fighter pilot and the family followed his postings around the world (until his dad's death in action in Kenya in 1953). His mother had often read Rudyard Kipling's Just So stories to him, and one day as he tried to decipher How the Camel Got His Hump alone in his cabin, the squiggles became transparent and lucid, and he found himself "inhabiting that private space with that wonderful synergy that exists between the book and the reader". Later, back in London, he was taken to a public library off Battersea Park Road and fell in love with Tove Jansson's Moomins.

But it was as a teacher in Oxford in the early Seventies (after his graduation from the university with a poor third, and after a while spent discovering that he was not the poet he thought he might be) that Pullman learned how to tell stories. Bard-like, he began telling, not reading, his class of middle-school children the Greek myths. The habit became a ritual, and he did it over and over again, with class after class, refining and honing his versions, learning what got a laugh and how to whip up suspense, and progressing from the gods to The Iliad and then to The Odyssey. It was a sort of apprenticeship: "Oh, it was an immense privilege," he grins. "I learned so much about myself, about my strengths and my weaknesses."

Young boys, he says, require a particular kind of literature to excite them. "And those books are being written. There's no shortage of stories for boys but they have to have access to them … We're back at libraries again."

The way Pullman the master story-teller tells it - and he finds himself becoming uncharacteristically "incoherent with rage" at the "under-valuation" of books by, as he sees it, a money and market-obsessed political elite - it is all colossally depressing. We appear to be in a vicious circle of poor childhood literacy and a diminishing supply of invaluable books.

Then again, perhaps tomorrow evening, on World Book Night, a perfect stranger will suddenly thrust into your hand a free book - maybe in the pub, or at the school gate or at the station, in a piece of charitable and social entrepreneurship which surely, paradoxically, defines the Big Society - and you can temporarily escape. Into a really good read.

World Book Night eve, tonight

What: A celebration on the eve of World Book Night with performances and readings from authors, artists and actors - bringing books to life for an audience of 10,000 people.

Why: The night is to launch World Book Night tomorrow - a nationwide initiative chaired by publisher Jamie Byng, right, in which one million free books will be given away by 20,000 readers.

Where: Trafalgar Square

Who: Hosted by Graham Norton and including performances from Margaret Atwood, Alan Bennett, Philip Pullman, Monica Ali, John Le Carré, Mark Haddon and Rupert Everett.

Who else: An audience of 10,000 people including 5,000 of the World Book Night book givers plus 5,000 members of the public.

Join in: The event is FREE and runs from 6pm to 8pm. Open access to parts of the square with full view of the stage for members of the public - just turn up.