There are at least as many tour guides in Oxford as there are colleges. Virtually every kind of introduction to the city is offered via the medium of enormous signs and enthusiastic hustlers whose patter would do credit to a Middle Eastern souk. If you want to tread in the footsteps of Lewis Carroll and JRR Tolkien, there’s a guided trip for you, and if you have children of a certain age then you, too, can wander around the halls, gardens and libraries that were used in the filming of the Harry Potter series. Yet nobody offers a less nostalgic tour, which has as much to say about present-day Oxford, and the rest of England, as it does about any fanciful and sanitised version of it. This would be a trip into the world of His Dark Materials, the series of novels by Philip Pullman that have transformed from a trilogy into, currently, a quintet with the publication of The Secret Commonwealth, one of the most anticipated novels of the year.