Harry Potter and the sad grown-ups

Walking through my train yesterday, staggering from my seat to the buffet and back, I counted five people reading Harry Potter novels. Not children – these were real grown-ups reading children's books. It was as if I had wandered into a John Wyndham scenario where the adults' brains have been addled by a plague and they have returned to childishness, avidly hunting out their toys and colouring-in books.

Maybe that would have been understandable. If these people had jumped whole-heartedly into a second childhood it would have made more sense. But they were card-carrying grown-ups with laptops and spreadsheets returning from sales meetings and seminars. Yet they chose to read a children's book.

I don't imagine you'll find this headcount exceptional. You can no longer get on the London Tube and not see a Harry Potter book, and I presume the same is true on the Glasgow Metro or the Manchester trams, or the beaches of Ibiza or clubs of Ayia Napa. Who told these adults they should read a kids' book? Do we see them ploughing through Now We Are Six or Five Children and It or even Tom's Midnight Garden? Of course not; if you suggested it they would rightly stare, bemused, and say: "Isn't that a kid's book? Why would I want to read that? I'm 37/42/63."

Nor is it just the film; these throwback readers were out there in droves long before the movie campaign opened. Warner Brothers knows it can't hope to recoup its reputed $100m costs through ticket sales to children alone. But the adult desire to tangle with Harry, Hermione and Voldemort existed long before the director Chris Columbus got his hands on the story.

So who are these adult readers who have made JK Rowling the second-biggest female earner in Britain (after Madonna)? As I have tramped along streets knee-deep in Harry Potter paperbacks, I've mentally slotted them into three groups.

First come the Never-Readers, whom Harry has enticed into opening a book. Is this a bad thing? Probably not. Ever since the invention of moving pictures, the written word has struggled to be as instantaneously exciting. Writing has many advantages over film, but it can never compete with its magnetic punch. If these books can re-establish the novel as a thrilling experience for some people, then this can only be for the better. If it takes obsession-level hype to lure them into a bookshop, that's fine by me. But will they go on to read anything else? Again, we can only hope. It has certainly worked at schools, especially for boys, whose reading has clearly taken an upward swing – for this alone, Rowling deserves her rewards.

The second group are the Occasional Readers. These people claim that tiredness, work and children allow them to read only a few books a year. Yet now – to be part of the crowd, to say they've read it – they put Harry Potter on their oh-so-select reading list. It's infuriating, it's maddening, it sends me ballistic. Yes, I'm a writer myself, writing difficult, unreadable, hopefully unsettling novels, but there are so many other good books out there, so much rewarding, enlightening, enlarging works of fiction for adults; and yet these sad cases are swept along by the hype, the faddism, into reading a children's book. Put like that, it's worse than maddening, it's pathetic. When I rule the world, all editions will carry a heavy-print literacy warning: "This Is A Children's Book, Designed For Under Elevens. It May Seriously Damage Your Credibility." I can dream, can't I?

The third group are the Regular Readers, for whom Harry is sandwiched between McEwan and Balzac, Roth and Dickens. This is the real baffler – what on earth do they get out of reading it? Why bother? But if they can rattle through it in a week just to say they've been there – like going to Longleat or the Eiffel Tower – the worst they're doing is encouraging others.

By now you're asking: "What's he got against these books, they're just a bit of escapism, just a great fantasy?" First, let me make it clear, I'm not here to criticise or praise the quality of JK's prose or inventiveness. They may indeed be the best children's novels ever written. But I'm sure JK would be the first to agree that they are children's books, that they are successful precisely because they appeal so directly to the childish imagination, address the problems and questions of childhood, enact the hopes and dreams of childhood. Now this is a completely different set of questions from those that mesmerise us in adult life. A child is free to wonder about magic, to believe in the clear purity of the struggle between good and evil, to bask in simple, unquestioning friendships. As adults, we deal with the constantly muddled nature of good and evil, we take on things like the constraints and longevity of love, we carry a responsibility for the safety of others, we crave success and fear failure, we confront the reality of dreams.

And this is why different books are written for these two tribes: there is barely any genuine or useful crossover between the agendas. When I read a novel, I look to it to tell me some truths about human life – the truths that non-fiction cannot reach. These might be moral, sexual, political or psychological truths and I expect my life to be enlarged, however slightly, by the experience of reading something fictional. I cannot hope to come closer to any of these truths through a children's novel, where nice clean white lines are painted between the good guys and the evil ones, where magic exists, and where there are adults on hand to delineate rules. Adult fiction is about a world without rules.

I know what you're thinking. You're thinking: "Does everything we read somehow have to improve us? Does even a novel have to 'enlarge' us? Isn't there room for a bit of escapism?" Of course there is! But there is such a thing as escapism for adults. There are plenty of books that have little or nothing genuinely to say about the human condition, but at least they are constructed from the building bricks of adult experience – there are sexual tensions in the evil, there is a dubiety between the good guys and the bad, there is an understanding of complex human psychologies. Even the flimsiest of science fiction or the nastiest of horror stories or the most intricate of spy novels uses this as the mortar to bond together its narrative. There is no such psychological understanding in children's novels: it would be foolish for any children's writer to hope that a child reader would understand, let alone enjoy, such a level of plotting. To read a children's book is not escapism – it's evasion, it's retreat, it's surrender.

So how do all these grown-ups manage to get through it? Of course, we have all read similar books out loud to our children and enjoyed the experience, possibly enjoyed the book itself – only because we were vicariously enjoying it through them. This is one of the few untouchable pleasures of parenting; to live and relive experiences through your children, whether book or film or music; to watch them discover the joy of listening. This is no different from taking them to see the latest Disney – you'll laugh, you'll get into it, you may even have a good time. But would you actually book a ticket to go and see it on your own? Of course not; it might be seen as rather sad, if not downright suspect.

So why do you read Harry Potter on your own? When the adult crossover first began, I remember a friend who works in the City covering his embarrassment by saying he had got so wrapped up in it while reading to his kids that he had to finish it alone in bed that night. At least, in those early days, he knew it was shaming to read a kids' book. Now we have the appalling spectacle of City brokers and merchant bankers block-booking seats in cinemas for their staff outings. God save us.

Is it just nostalgia? For those of us old enough to have been brought up in a largely literary age, where child escapism existed mainly on the page, Potter might be seen as a return to Narnia and Dolittle and Streatfield. It seems as though there has been nothing quite as good since – but that's only because you're supposed to grow out of children's books.

For others, no doubt, brought up in the Star Wars age, it is yet another nostalgic return to England-land. There is no denying that Rowling has gone out of her way – maybe not cynically, maybe by genuine heartfelt choice – to place Potter-land in the traditional English milieu, all green fields and mossy stone quads, something more English than anyone under 80 has ever known.

Sure, maybe Harry Potter does have all these side values; it's safe, it's England, it's like something we used to read. But get real, please, there is so much good fiction out there, written specifically for your adult age group, written with you in mind. Please, next time, choose that. Don't keep running away from life.