Ask any 13-year-old with a passion for computers what he wants to do when he grows up and the answer will be predictable. “Something to do with computers,” he will say. “Yes, definitely computers.” But when Luke Johnson’s mum tentatively suggests that he ought at least to consider other options (becoming an author, for instance), his response is surprising. “An author?” he repeats, and thinks about it for a twentieth of a second. “I already am one.”

And it’s true. For here he is, sitting on a swivel chair in his publisher’s Islington office, answering questions about his new paperback and posing patiently for press photographers. Admittedly, he has just discovered that by yanking one of the levers under the chair he can make the seat drop suddenly with a dramatic swish and, from time to time, he can’t resist it. But the boy is 13, after all.

Not that you’d guess it from his book. Freaks, Geeks amp; Asperger Syndrome is a cool and confident work that belies its author’s youth. The experts reckon that Luke has a reading age of 18-plus (“I’ve read every single book in the house nine times over,” he says), but most people that age would be hard-pressed to produce such witty, effortless prose.

It was Luke’s idea to write a book for and about children with autism and related disorders - a book that, in the words of the subtitle, would be “a user guide to adolescence”, while at the same time giving parents and teachers the sort of insight into these conditions that could only come from an author who was himself diagnosed with Asperger syndrome.

“I thought of it, but I didn’t do anything about it for a long time,” says Luke. “Then mum told me to stop being lazy and get on with it.”

They worked on the structure of the book together, planning chapters about the various autistic spectrum disorders, about coping with school and dealing with bullies, about making (or not making) friends, even about dating (or not dating).

They mapped in lists of tips for children (“Listen carefully when you are being taught”), advice for teachers (“Don’t announce in class that someone is being bullied”), and guidance for parents (“A child on the autistic spectrum needs things spelling out to them more than most. In a way they are like foreigners”).

Then, during a long summer holiday at home in Blackpool, Jacqueline Jackson typed while her son dictated. At the end of the nine weeks (Luke goes to a fee-paying school now, having been driven out of the local comprehensive by persistent and sometimes violent bullying) they had 50,000 words and a deal with Jessica Kingsley Publishers.

So what sort of child spends his summer holidays rapping out the kind of factual text that his teachers might want to read? A bookish prig? A precocious swot? An insufferable pain?

That’s undoubtedly how Luke is seen by many of his peers, and his detailed descriptions of bullying make chilling reading for anyone, with AS or not, who preferred the library to the playing field at school. But, from an adult standpoint, the boy who swishes up and down on his publisher’s chair is a likeable, outgoing kid.

Beneath the gelled, spiky hair, he has the elfin looks that are strangely common among autistic children. And although he sometimes talks with startling certainty (if you say something he doesn’t agree with, he will tell you so unhesitatingly), the jokiness that permeates his book is equally endearing in conversation.

His sense of humour improved, he says, when he started on the casein- (milk protein) and gluten-free diet that many autistic people find beneficial. “The diet helped to remove the fog,” says Jacqueline, who volunteers on a parents’ helpline and is planning a PhD on the links between diet and autism. And, although Luke has a tendency to take things literally (very Asperger), he is clearly overcoming this by teaching himself how the “normal” world uses metaphor and simile.

He does carry in his head an alarming quantity of technical data about computers, but he is learning to keep it quiet, while reminding himself (and anyone else who will listen) that most boys of his age know rather too much about football.

Most astonishing, perhaps, is Luke’s knowledge of the various developmental disorders dealt with in his book. Yet this is hardly surprising when you consider that three of his six siblings have been diagnosed with autism, dyspraxia or attention problems.

Not that Luke particularly likes terms such as “problem” and “disorder” in this context. “It’s fun being weird,” he says. “That’s the AS motto. Different is cool. Every book I’ve read by an AS person has that message.”

And it’s a message he would like to get across to teachers, who he believes should be more accepting of the differences between children. “A teacher should never push an AS person to engage in social chit-chat, because that just is not what they are,” he says. “AS people are different, and teachers should just accept that.”

It’s this positive - almost celebratory - view that could well make Luke’s book a favourite among children, AS and otherwise, who find themselves out of tune with their classmates. But it would be a mistake to think that its young author has life entirely sorted.

“I have a hang-up sometimes that no one will ever be friends with me,” he confides.

And then, with a nonchalant swish, he is bouncing on his publisher’s chair once more.