Even without his tarantula, there's something rather creepy about Darren Shan. So when the little fella in the front row asked him if he was "a real vampire", a perceptible shiver went round the audience as the author grinned broadly and asked the boy his blood group.

Shan (real name Darren O'Shaughnessy) was at the book festival at the weekend to push his second children's book The Vampire's Assistant (HarperCollins £3.99). The first, Cirque du Freak, was published in January to huge acclaim. "Happy New Fear," proclaimed one newspaper.

It's the quasi-autobiographical tale of "an ordinary boy" whose fascination with spiders draws him to an illegal old-world freak show. He walks into a bizare twilight world which appears to be a replica of the Victorian stereotype: there's a bearded lady, the Wolf Man, the Snake Boy, and Rhamus Twobellies, who lightly snacks on cutlery and glass. Among the performers is the sinister Mr Crepsley and his lethal performing tarantula, Madame Octa. Darren and his pal Steve can't anticipate the price they will pay to see the circus.

Within a month of publication, 9000 copies had walked off the shelves, mainly into the hands of boys aged 10 to 15. Comparisons were soon being made with Joanne Rowling, especially when Warner Brothers snapped up the film rights.

While Rowling was toiling in cafes in Edinburgh, Shan had been beavering in his small bedroom near Limerick. At 27, he survived financially only by living with his parents.

By chance Shan and Rowling share the same agent, Christopher Little. Shan had decided to try a children's book as light relief, while wrestling with an adult horror story. "When I showed the manuscript to Little, he didn't say much, just flashed me a big smile," recalls Shan.

Little sent the manuscript to Rowling who was wildly enthusiastic. "Full of satisfying macabre touches, it explores the powerful fascination of the dangerous and unnatural and also, movingly, the obligations of friendship," says Rowling.

As with Rowling, Shan plans a series. "I've already written most of the first nine and plan around 24," he says. The third, Tunnels of Blood, is published in November.

Though vampires age at only a fifth the normal rate, eventually Darren will hit "vampuberty". Like the Harry Potter narrative, the challenge then is to write in such a way as to carry your original readers while remaining accessible to a new generation of 10-year- olds. In other words, Darren's sex life will be remaining rather sketchy.

Another similarity is the juxtaposition of the macabre and the mundane: the dark spooky world of the circus and the conventional daytime existence of a boy who initially, at least, goes to school, plays football, and fights with his kid sister. At the end of the first book Darren becomes a half-vampire, a neat device, enabling him to bridge both worlds.

Shan admits to an obsession with vampires stretching back to childhood: "Other kids had pop stars and football teams on their bedroom walls. I had Dracula."

He grew up with Hammer Horrors and indy comics. "I was a deprived child: there were no Goosebumps or Point Horror in those days. That's why I wanted to write such stories."

He delights in the subversion of stereotypes: "I wanted to use vampires but if they went around killing people to suck their blood, they wouldn't survive. They would be arrested." So his vampires deftly knock people unconscious by breathing gas on them, then sup their pint of plasma, before healing the wound.

And the circus performers, instead of pathetic objects of derision, are magical beings with superhuman powers, such as Hans Hands, who can run 100 metres in eight seconds on his hands.

"In the original shows, these people were ogled and laughed at. It was disgusting. In my books, they are celebrated rather than exploited. So far nobody with a disability has objected. I'd be surprised if they did," he says.

He found writing for children harder than he'd expected. Though closer to his own childhood than most children's authors, it took him several months to adjust his narrative voice.

"At first, I made the characters too juvenile. And I wasn't sure how much gore children could take, so I sent part of the manuscript to the school where my mum teaches, along with a little questionnaire. The forms came back saying they loved the story but wanted more blood!

"Children love being frightened by books and films. They enjoy the creepy feeling because they know it won't hurt them. Some people ask if I'm not afraid of giving children nightmares by including scenes like the woman having her hand bitten off by the Wolf Man, but I just laugh. Horror stories are meant to give you nightmares, That's part of the enjoyment."

Not all reviewers raved about Cirque du Freak. Some found the characters shallow and thin. Shan admits his writing is short on detailed description but says that's part of the genre: "Everything is pared right down. You have to engage the sympathy of the reader but the essential thing is to keep the narrative bowling along."

Of course, Darren is just an ordinary chap in real life, isn't he? Yes, of course. The tape of our interview turned out to be blank when I tried replaying it. Technical foul-up, I guess. Anyway I can always use my notes, except . . . well, when I opened my bag last night, the notebook wasn't there. Don't imagine I'm scared or anything. It's just . . . Well, I have a rather rare blood group.