Harry Potter and the Half-Blood Prince, by J K Rowling

Harry in the night

The "geeks" laughed and cheered. This street is full of bars. After nine, it belongs entirely to girls with orange skin and boys with no chance. On any other night, Veronica and her friends would not be welcome and would not want to be welcomed, but tonight they have taken control of the top right-hand corner. It's Geek Alamo.

It struck me how like the core Potter story this little scene was: a group of sensitive children, marginalised by bullies and bores, finding glory and each other in the land of reading. It also struck me that the original Potter generation has grown up. Of course, adults have always read the books, but now the original readers are on the brink of adulthood themselves. Veronica was in the same class at primary school as my son, Aidan. They both read the first Potter book when it came out in hardback. They were in Year 4. Veronica wrote to JK and JK wrote back! She said Veronica had made her day.

Years later, Veronica wrote to Warner Brothers, telling them - quite rightly - that she would make the perfect Hermione. They knew Harry before he was a phenomenon. For them, the night's events are not so much a hype as a vindication. And now here she is: a young woman about to apply to university. Hadn't she grown out of it?

Not really. She had spent the last few weeks logging onto the Leaky Cauldron website, tuning into rumours and speculation. "When the next one comes out, that'll be the last one. I'll be doing finals. I'll have no choice but to grow out of it then." Tonight she will go home, cosy on down and let Harry take her forward into adventure, but also back into the comforting atmosphere of year four and that first touch of Harry in the night.

One of the harshest moments in all literature is when C S Lewis's Aslan tells Susan that she is never coming back to Narnia (because she's wearing lipstick). Part of the power of the Potter series for Veronica and Aidan's generation is that, every few years, the door re-opens and they are readmitted to the magic of their own childhood. It's a measure of Rowling's mastery that she can pull this off. When George Lucas tried to do the same with Star Wars, he produced something that made you want to get in touch with your own inner child and put him into care.

So I was envious of Veronica's return ticket to remembered enchantment, but also troubled. Why didn't my Aidan want to do the same thing? Why was he partying in Garlands instead of queueing here with the rest of us? What was wrong with his childhood that he didn't want to re-live it?

Back home, there were plenty of people reading. My eight-year-old has never read the Potters before. He's speed-reading the first five so he can start the sixth. My wife missed HP5 and is reading that. Only my daughters refuse to put down their Eva Ibbotsons. I curled up on the couch with the Half-Blood Prince, intending to read until Aidan came home.

Coming at the book fresh from all those primary-school reminiscences, the first surprise is how modern it feels. I've always thought that one of the reasons the books were successful was that Rowling was somehow able to keep all the fabulous panoply of generic fantasy without having everyone "thee" and "thou" like a bunch of mumming Yodas. Part of the magic of Potter is that, beneath the patina of nostalgia and tuck, there is a very modern sensibility. For instance, that brilliant invention, Quidditch, is really a computer game with jolly-hockey sound effects. The same goes for the Sorting Hat.

Indeed, the way the plots of the books work is sometimes more like a PS2 game than a novel, with Harry ordered to collect various tokens, solve puzzles and move onto new levels. But the new book engages with the present in a more direct way. It opens with a twitchy, exhausted Prime Minister closeted with secret advisors, trying to deal with a run of bad news - some horrible murders, an unexplained civil disaster, freakish weather.

Voldemort and his forces are making random attacks on wizards and Muggles alike. The Ministry gives out security leaflets and starts making pre-emptive arrests. When Stan Shunpike is picked up for mouthing off about the Dark Lord, Harry says it's silly but Hermione says that "they have to look as though they're doing something". Half the shops in Diagon Alley are closed. Others remain defiantly open. It's a startlingly accurate description of London after the bombings.

When I read the first Harry Potter, I found the Hogwarts house system irritatingly contradictory. Slytherin is a house for bad wizards, so why does Dumbledore agree to educate them? So they can hurt him more knowledgeably? It didn't seem to make any sense. In other fantasy books the enemies are a different species or, at least, from a different country. But this social mingling of the good guys and bad has become one of the most powerfully truthful and radical aspects of the books.

Whoever you think the real enemy is - whether it's the affable classroom assistant who turns out to be a bomber, or the neighbour who gives you a cheery wave while knackering your planet and menacing your child with his eejit SUV - they live nearby. And the truth is that we mostly rub along with them.

The boy who yelled "Geeks go home" came back half an hour later and had a chat with us. He was in the same class as one of Veronica's mates. He tried to get me to buy him a copy. Yes, he's the enemy, but he's mostly alright really. Until the chips are down, we'll mostly just mark our territory with half-meant banter and get on with it.

The atmosphere of this book is as sweaty and paranoid as any thriller, but the characters behave like we do. They have suspicions but are too polite to act on them. They have fears but keep them to themselves for the greater fear of being thought cranky. When the casualties are named, they are always friends of friends. When the enemy is revealed, it's horribly close to home. It is all queasily real in a way that Ian McEwan's vicarish attempts to get with it in Saturday simply are not.

Gripping as the new book is, I must have fallen asleep. When I woke up, Aidan was home. He was lying on his bed clutching my copy. He was already on page 200. So that's all right, then.

Frank Cottrell Boyce's novel 'Millions' (Macmillan) has won this year's CILIP Carnegie Medal for children's literature