On another planet: Patrick Ness on despotism, "young adults" and the thrilling denouement to his Chaos Walking trilogy

Patrick Ness's latest is the thrilling conclusion to his Chaos Walking trilogy for young adults. Not that he works with any market in mind – all the better to provoke the strongest reactions in the strangest places

(KALPESH LATHIGRA)

My aunt phoned yesterday. She's finished the first two books in the Chaos Walking trilogy and oh-my-God she LOVED them and wants to know when she can get her hands on the third. The visitor comments on Patrick Ness's website suggest that she's not alone. The Knife of Never Letting Go and The Ask and the Answer both end with evil cliff-hangers, and there are many nervous people around the world counting the days until book three. But the wait is nearly over. Monsters of Men, the trilogy's extraordinary conclusion, hits UK shops tomorrow.

As we walk over to meet Ness, our photographer asks what his books are like and I can't manage an adequate answer. Being ambitious, capacious books, any attempt at description feels reductive. Yes, they're set on another planet, but that makes them sound like the kind of fantasy they're not; they're "high-concept", packed with original ideas, but they're not gimmicky, nor do the ideas weigh them down or hamper their pace. They make readers question our own society – political power, the twin threats of despotism and terrorism, mass communications, torture – but above all they're gripping, thrilling, hurtling stories in which the brilliant ideas serve a great plot and strong characters, rather than the other way around.

"I think the reason teenage fiction is so popular with adults is that adults hunger for narrative just as badly as teenagers do," says Ness. And so it has proved. One friend, a 40-year-old lawyer, was on the Tube when he reached a certain (now infamous) scene and had to grab a handrail to steady himself for fear of passing out.

This strength of story goes some way to explaining Chaos Walking's massive success. Pleasingly, those website comments include many who are discovering the excitement of getting lost in a book for the very first time. And along with the popular acclaim, there has been recognition from every major award Ness is eligible for: the Booktrust Teenage Prize (Knife won, Ask shortlisted), the Costa (Ask won), the Carnegie Medal (Knife shortlisted, Ask currently shortlisted) and The Guardian First (Knife won and you're not eligible for a second). So, just a little pressure on book three?

Readers will not be disappointed. The final stretch of Monsters of Men is perhaps the most perfect conclusion to any big series I've read. It comes as no surprise that Ness had this stunning climax – pulse-racing, tear-jerking, mind-boggling – in his sights all along. "I like to know where I'm going. I had the last line of all three books before I started the first line. How you leave the reader is so important – not the climax, I call it the 'exit feeling'. I couldn't wait to get to it." The first line of the trilogy, incidentally, is: "The first thing you find out when your dog learns to talk is that dogs don't got nothing much to say."

"I read that first sentence and knew I had something different," recalls his editor, Denise Johnstone-Burt. "Every sentence that followed confirmed my first impression."

American-born Ness, now London-based, was the author of two critically acclaimed books – a novel and a collection of wonderfully strange, spikily funny stories – before he embarked on The Knife of Never Letting Go, which launched him so explosively into the "young-adult" market. But at first, he didn't know that a "young-adult" novel was what he was writing. "It started as just a story, and as I slowly found the voice, and figured out what the themes were, it sort of made itself apparent as a young-adult story. And I thought, there's freedom here, too. With teenagers, if they don't like you, they really won't like you: they'll close the book right up and tell all their friends not to read it and how stupid you are. But if they do like you, then they're not snobs and they'll follow you much further than an adult reader will."

Even now, though, Ness doesn't set out specifically to write young-adult novels; he resists such constraints to his writing process: "If I sit down to write a young-adult novel, then I'm going to write either to the punch-pulling expectation of what I can't do, or I'm going to go the other way and think about what can I sneak in to be 'down with the kids' – which would be excruciating. So the best thing for me is just to ignore all that and write a story that I want to read. If you set out to write an adjective novel, you're setting out to write a mediocre novel; your allegiance is to the adjective, not to the story, and then that just sucks all the joy right out of it."

"Joy" is a word that crops up often when Ness talks about his work, and the work of writers who excite him. "I talk about this ad infinitum, the need for a writer to have joy in what they're writing. I think the reader can tell. So I look for a writer having a great time, who wants to share their great time."

In a series of writing tips produced for Booktrust (he was their first online writer-in-residence), Ness wrote that novels "eat ideas like forest fires eat trees". So he's careful to keep his own narrative fires well-fuelled. Of the many imagination-stretching ideas that fill and colour Ness's books, the most fundamental is Noise. Noise is thought made audible, where everything happening in your head can be heard, all the time.

"The world is already Noisy. Teenagers today have less privacy than anyone has ever had: everything they think is online; every stupid thing they've done has been filmed and put on to YouTube. I think that's costly – teenage years are when you really need privacy the most, when you're figuring out your boundaries and crossing them, usually. And so I just worried: what would it be like if you really had no escape, and if everybody knew what you were thinking? How awful that would be if you were a teenager." Ness's success in capturing that transitional teenage experience is central to the book's appeal.

Unlike the men of Chaos Walking, the women have no Noise. The series begins with teenager Todd – who believes that all the women have died – stumbling into a patch of silence that turns out to be a girl, Viola, newly crash-landed on his world in a spaceship. So why the men and not the women? The books, says Ness, "are all about difference. I think our biggest failing as a species is the inability to see difference as difference – it's either better, so we have to pull it down, or it's worse, so we can exploit it. Difference is rarely ever just difference."

Throughout the trilogy, Ness spins a web of profound moral complexity, broaching subjects that interest and trouble him. "Saying I don't set out to write young-adult novels is not an abrogation of responsibility. I'm a moral person. Moral issues are interesting; it's never cut and dried." Not offering answers, then, but raising questions? "I want [readers] to think," he says, "and I want them to think 'What would you do here?'" So when the books explode into civil war, "It seems like we should know who the good guys are, but..."

The moral complexity isn't lost on Ness's young readers, either. When I ask 12-year-old Charlie his favourite thing about Knife, he replies : "In every book I've read, when aliens land on another planet they are always the bad guys and the people living there are the good guys, but here you don't know who the monsters are."

So after this huge, bold undertaking, what next? Ness's publishers have offered him a potentially delightful challenge. When the children's author Siobhan Dowd died in 2007, she left preliminary notes for a novel, with only the first few thousand words drafted, and Ness has been invited to write that book. He's not trying to mimic Dowd's writing, but to take her "terrific basic idea" and, with "energy and momentum and mischief", try to "write a book I think Siobhan Dowd would have liked. It feels like being handed a baton." He discusses this book with the same enthusiasm as when he's talking about Chaos Walking – the same joy, perhaps. He makes the challenge sound really, really fun.

Go to independent.co.uk/books to read a longer extract from 'Monsters of Men' and to win signed copies of the book