Sending a journalist to interview her childhood hero can make for biased copy. Faced with an idol the spirit of journalistic enquiry crumbles. The normally incisive reporter finds himself asking: “So, were you always this great?” while the last ten minutes of tape record the star-struck journalist producing various first editions, album covers, and paper napkins for signing.

Waiting for Paul Stewart and Chris Riddell, the author-illustrator team behind the hugely successful series of children’s books The Edge Chronicles, I try to keep this in mind, but when Stewart rings to say that they will be late I carefully save his number. Not because I plan to call him but just because... he’s my hero.

I was nine when Beyond the Deepwoods was published. The tenth and final instalment, The Immortals came out last year. I have invested thirteen years of my life in the Edge. So, when it comes to interviewing Stewart and Riddell, I fall foul of every rule I’ve set. I produce a first edition Deepwoods for signing, I tell Riddell that I’ve bought four of his drawings, I ask which their favourite book is only as an opportunity to launch into a eulogy of mine. Mercifully they are used to this sort of thing. They have just come from a signing at The Leys School and are practised at dealing with nine-year-old fans.

“That’s the tragedy of childhood: always being disappointed by your own wardrobe.”

The Edge, like The Hobbit and The Lord of the Rings, started with a map. A map of a promontory, a great flint of a world jutting out over an abyss. A world populated by extraordinary flora and fauna: banderbears, wigwigs, wood trolls, hammelhorns and The Edge’s own he-who-must-not-be-named, the gloamglozer. All are drawn with taxonomic precision by Riddell. For writer Stewart, Riddell’s Moleskine sketchbooks “are almost like natural history books. It’s as if I’m looking at something that already exists.”

The Edge’s ancestors are Tolkien, Narnia and Mervyn Peake’s Gormenghast. They are both devotees of Peake, “In The Edge Chronicles, there’s more than a dollop of that sensibility, that visual fantasy. My favourite review ever was ‘This is Gormenghast for kids’.” On Tolkien they are divided; Riddell is a fan, Stewart less so. In the case of Narnia, their allegiances are reversed. “I know you loved it, your whole wardrobe business,” Riddell reminds Stewart, “that thing of going to it and saying: ‘please let there be a world on the other side’. Most of your childhood was spent in some sort of cupboard or another...” Stewart, who has a drier sense of humour than the effusive Riddell recalls “That’s the tragedy of childhood: always being disappointed by your own wardrobe.”

As for Riddell, he works in the great tradition of black-and-white illustrators: Tenniel, Rackham, Heath Robinson, Beardsley, E. H. Shepherd, Searle. Looking further back, Riddell cites Dürer, Rembrandt and Goya as influences. Not to be outdone, Stewart mentions that he likes the Chapman Brothers.

Like couples who have long been married, Stewart and Riddell have a joint memory bank. They finish sentences, defer on facts, and know what the other will have to drink. They met eighteen years ago, at the nursery school gates. “Our sons were two.” They were introduced by presumptuous mutual friends who said to their wives, “Your husbands do the same sort of thing. Why don’t they get together?”

Stewart invited Riddell to breakfast. “Was it breakfast?” “At first, yeah...” “I thought it was a morning coffee or something...” “And I left a sprinkling of books out....” “He had one on every stair. What a big head.” Then they ran into each other at a publishing party in London. Riddell recalls the meeting: “Our eyes met across a crowded room. It was one of those moments...” “I had met various illustrators, but not mentioning any names, they weren’t any good, and then I met Chris...” “Well, the point is: we met each other.” They took the train back to Brighton together and eighteen years, and two million UK copies later, here they are. They now live on the same street.

After nearly two decades of The Edge, which ended with a cliff-hanger - literally, our heroes are dropped off the edge into the unknown - they are struggling to let go. The adventure continues in weekly chapter installments on their blog Weird New Worlds. “Basically it’s therapy,” Riddell confesses, “treating our withdrawal symptoms. We can’t just go cold turkey.” Now they are working on a series of books about “pioneer America with dragons.” There is strictly no map.

The last fifteen years have been a golden age of children’s literature: Harry Potter, Northern Lights, Lemony Snickett, The Edge. I ask who they think will last the course. “Pullman will be read in 100 years time, I think perhaps Harry Potter won’t. It is too much of its time. A bit like Enid Blyton. Rowling will be seen as this extraordinary phenomenon, but people will see her influences more clearly. Someone like Philip Pullman is an original.” And as for The Edge? Riddell weighs in: “I’d like to think that as far as The Edge is concerned, it will be a little footnote and people will say ‘What were they doing?”

Publishers thought that The Edge could not be done. Books for the over sevens just weren’t illustrated, and Stewart and Riddell wanted to produce illustrated books for readers aged seven to twelve. The gamble paid off. The books sell in the US, Germany, Japan, Portugal, Brazil, Serbia, Montenegro, Lithuania. “In terms of translation rights,” Stewart wryly observes, “the breakup of Eastern Europe was extremely good for us.” Their publishers also said that the series would never appeal to girls. “We are so glad to have met you,” Stewart says as they leave, “so glad you are a girl.” And, as I watch them walk off into the sunset, clutching my Deepwoods, I do feel like a girl, aged about nine. That’s the power of great children’s literature – you never really outgrow it.