The Immortals is the tenth and final instalment in the Edge Chronicles series - can you tell us what it's about in a few sentences?

The Immortals is set 500 years after the end of the previous book, in the Third Age of Flight. This third age has been made possible by the unlocking and harnessing of stormphrax's immense power: the power of lightning.

Phrax crystals power the mighty sky vessels that trade between the three major cities of Great Glade, Hive and Riverrise, and enable their intrepid captains to travel to the furthest reaches of the Edge.

We always intended the Edge Chronicles to be a self-contained series of books, and The Immortals completes the story arc. Various threads were left untied in the previous books. What happened to Cloud Wolf in the white storm? Was Twig alive or dead when the caterbird takes him to Riverrise? What happened to old Sanctaphrax when the anchor-chain was cut and it floated off at the end of Midnight over Sanctaphrax? Where did stone sickness come from? And what became of the gloamglozer? All these questions, and more, are answered in The Immortals .

Do I have to read the other Edge Chronicles books to understand what is happening in The Immortals?

Like all the other books in the series, The Immortals is a stand-alone read. No prior knowledge of the Edge is necessary to read and enjoy it, although of course the more books that the reader explores, the deeper his/her knowledge of the Edge world will become.

Why is it called The Immortals?

You'll have to read The Immortals to find out!

Do we meet any new characters/monsters? How do you dream up your characters?

As well as becoming reacquainted with old characters from the Edge, we meet a whole range of new characters. Nate Quarter, a young phraxminer, his friend, Slip, a grey goblin scuttler and Eudoxia Prade and her prowlgrin, Majestix. Weelum, the city-dwelling banderbear. Kulltuft Warhammer, the brutish High Clan Chief of Hive. Golderayce One-Eye, an ancient waif, the Custodian General of the Riverrise spring…

As for creatures, we encounter many. Some we have not seen before, like the ghost quarms and lanternjaws, and the scavenging crimson-winged hammelbills that roost in the Midwood Decks. Some, however, we have already met in previous books, like the ferocious wig-wigs that can devour any hapless banderbear that falls into their clutches in a matter of seconds. As with all the other characters and creatures in the Edge, some come from Paul's descriptions while others start off as rough drawings in Chris's sketch books.

How did the idea for The Edge start?

The Edge Chronicles themselves started in one of these sketch books. Back in 1994, Chris drew a map of the Edge, with its familiar jutting rock, floating city and endless forests. He gave it to Paul, saying 'Here's the world. Let's find out what happens in it.'

What is your favourite character you have created and why? Paul's favourite character is Xanth Filatine. He is a complex figure, with divided loyalties, sometimes doing good things for bad reasons, sometimes doing bad things for good reasons.

Chris's favourite character is Zelphyius Dax, a librarian knight of the Third Age of Flight, who voyages through the Deepwoods aboard his skycraft, the Varis Lodd. He remembers and reveres the past, and is an opponent of new phraxships and the ecological damage inflicted by progress.

Is there any episode/journey/battle in the Edge Chronicles that you would have loved to be at/take part in and why?

Paul's favourite episode comes from the Last of the Sky Pirates. He would have loved to learn how to make his own varnished sumpwood skycraft alongside Rook, Magda and Stob in the woodtroll workshop, the Gardens of Light, the slaughterer's camp, and taken the finished craft up into the air above Lake Landing.

Chris's favourite episode comes from The Immortals and is supper at the School of Restoration of the Sumpwood Bridge Academy, one of the most magnificent buildings in the Third Age. The food was good too!

Both of them would have loved to have good seats at the Thousandsticks Stadium in The Immortals to watch the Reckoning, where twelve teams of a hundred players each, representing all the districts of Great Glade, gathered for their annual thousandsticks match final.

How do you decide where to put the drawings?

In the books Paul and Chris read as boys, the colour plates were often nowhere near the text they described. In the Edge Chronicles, we've taken care to match illustrations and text. If we read about a new character, we also see him. Buildings, cities, sky ships and creatures are depicted both in words and pictures. Only when an illustration might give too much away, are they put after the text, so that, on turning the page, a dramatic scene or revelation can be seen.

Does Chris do the drawings first and then Paul writes the story or the other way round?

Our working method varies. Sometimes passages influence the drawings, sometimes the drawings influence the text as we are working. Most important, however, is talking. The Edge Chronicles are a collaboration. We plot and plan together, talking over every aspect of the storyline and the Edgeworld itself. Out of these long conversations, the books slowly emerge, first as text, and then final illustrations are added.

Do you always get on well with each other?

We never argue about the books, though some of the conversations can become quite impassioned as we both argue our corners. For both of us, the quality of the finished books is the all-important criterion. We met sixteen years ago at the front gates of the nursery our respective sons went to, and since then have become good friends. Our kids went to the same schools, our wives are friends, and we are near neighbours, which makes it very easy for one or the other of us to drop in on the other with a finished chapter or a bunch of pictures.

Do you like writing about the battles and action scenes - they're always brilliant?

We love immersing ourselves in the Edgeworld, but the battles and action scenes are perhaps the most exciting to write.

Paul particularly enjoys the scenes that involve creatures, like the hoverworm attack in Beyond the Deepwoods, the swarms of predatory snickets in Last of the Sky Pirates, the rock demon and rubble ghouls of Vox and the fearsome creatures of the Wig-Wig Arena unleashed on their hapless victims in Midnight over Sanctaphrax.

Chris loves military history, and brings a knowledge of battle formations and tactics to the books, both in the land battles of Freeglader and the sky battles of Clash of the Sky Galleons.

Chris, what is your favourite bit of the Edge to draw - the maps, the characters, the action scenes or something else?

Chris has always loved drawing battle scenes, and these are among his favourite illustrations in the Edge Chronicles. The Bloodbath on the Blackwood Bridge from Vox and the Battle of the Midwood Marshes from The Immortals were two of his personal highlights.

Would you describe the Edge Chronicles as traditional fantasy?

The Edge Chronicles are not traditional fantasy. They are influenced by the tales of the Brothers Grimm and Mervyn Peake's Gormenghast. In much traditional fantasy, a world of good versus evil is depicted. The Edge does not deal with black and white in this way, but rather in shades of grey, which is far more like our own world. There is also no magic. We thought it would be too convenient if a cloak of invisibility or magic spell was used to solve a problem. Instead, the world has its own physical properties, from floating rocks to solidified lightning.

Your books are published as children's books, but they also appeal to adults. Do you set out to write for a specific age group, or do you hope that it will appeal to all?

We write the Edge Chronicles for ourselves, or rather the twelve-year old boys we once were. Both of us loved adventure books when we were that age, from Henry Treece to Willard Price. We'd have loved the Edge if it had existed then! Throughout the writing of the Edge series, both of us have had long, detailed conversations with our sons about the world, and their reactions have helped us steer a course through the books. Our readership is very varied, from enthusiastic boys and girls and their parents, to a post-graduate student in Los Angeles who was writing his thesis on the Edge. And Chris's mum, a vicar's wife in her seventies, also loves them.

Did you enjoy writing/drawing when you were younger, and how old were you both when your first book was published?

Paul loved writing when he was a boy, and English was always his strongest subject, both at school and university. At the age of ten he read Norton Juster's The Phantom Tollbooth and, since there wasn't a sequel, decided to write one of his own. It was this book that inspired him to become a writer. His first novel, the Thought Domain, was published in 1988.

Chris has always loved drawing, and was delighted to run away to Art School at the first opportunity. He also loved books and reading as a child, and becoming a children's book illustrator combined drawing and reading. His first book was published in 1986, and was called Mr Underbed.

Did you imagine when you started the Edge, that it would turn in to 10 books?!

When we first started the series, we thought it might turn out to be a trilogy - if we were lucky. By the time we'd finished the three books about Twig, Beyond the Deepwoods, Stormchaser and Midnight over Sanctaphrax, we had so many ideas remaining that we decided to produce two more books - the first, Curse of the Gloamglozer, a prequel, to tell the tale of his father, Quint; the second, The Last of the Sky Pirates, a sequel, to reveal what had happened to Twig. This book introduced a third main character, Rook Barkwater, Twig's grandson. His adventures also turned into a trilogy, with Vox and Freeglader.

So both Twig and Rook had three books each about them, but Quint only had one - though not for long. The Winter Knights and Clash of the Sky Galleons followed his boyhood through the Knights Academy of Sanctaphrax and off in the Galerider in search of his family's murderer. The Lost Barkscrolls is four stories in one book, taken from episodes that occurred in the first and second Age of Flight. Once we had got so far, The Immortals - the tenth and final book - had to be written to bring all the threads of the stories together and, as American therapists put it, to achieve closure.

Is there a film of the Edge Chronicles under development? And if so, which actors would you want to play the main characters?

There has been a lot of film interest in the Edge ever since the first book was published, and indeed the series has been optioned a number of times. But it needs to look as good as Lord of the Rings, and that means a huge budget.

Twig, Quint and Rook would be brilliant child actors in their first starring roles, with Tom Cruise as Wind Jackal, Colin Farrell as a grown-up Quint, Meryl Streep as Welma, Angelina Jolie as Varie Lodd, Ian MacKellern as Linius Pallitax, Most High Academe, and Alex Jennings, who reads the audio books, as Zelphyius Dax.

What books did you both like to read when you were younger?

Paul loved Rupert annuals, the Phantom Tollbooth by Norton Juster, the 'Alice' books, all the novels of Alan Garner, especially Elidor. He also read huge amounts of science fiction. Chris, as a boy, loved Flat Stanley by Jeff Brown, old Dandy and Beano annuals, Professor Branestawm by Norman Hunter, and the Weirdstone of Brisingamen by Alan Garner.

You travel the world doing talks and signings about your books - what's been your funniest story from the tours?

Lots of funny incidents have occurred while we've been on tour. Like the time in Dublin when Paul was about to read from an advanced copy of a book to a bookshop full of excited kids, only to have it taken from his hand by an interested customer, who then walked off with it. Or being asked to sign a life-size hand-coloured photograph in Dayton, Ohio, which made us look as though we were wearing fright masks. Then there was the photo-booth in Seattle, the throwing-up incident in the gym in Chicago, the grumpy schoolgirls in Brisbane, being driven by an orc in New Zealand, and morning TV in Canada…

What is your favourite food?

Paul - Marmite and marmalade sandwiches. Chris - My mother's South African lamb and egg curry.

What is your idea of a perfect Sunday?

Paul. In winter, a long walk on the Downs with my wife, or the kids, or friends - or all of them. Then roast and a good dvd in the evening. In summer, a swim in the sea and a picnic on the beach. Chris. Reading the Sunday papers in bed. Listening to the Archers in the morning and watching a good game of rugby in the afternoon.

Best and worst holiday?

Paul. My best holiday was in Australia. I visited a place south of Adelaide called Kangaroo Island, a wonderful place where the animals - seals wallabies, possums and koalas - had no fear of humans. My worst holiday was a family holiday in Plymouth when I was a boy. It rained non-stop for two weeks.

Chris. My worst holiday was on a canal boat when I was twelve and I got my leg crushed in a lock. Nothing broken, but badly bruised, and I was on crutches for two weeks. My best holiday was in South Africa with my wife and kids. We visited Boulders Beach and swam with African penguins who nest there. The penguins appeared in Fergus Crane as Finn, Bill and Jackson.

Best lesson at school and the worst?

Paul. My best subject was always English, both reading and writing. My worst was maths. I never grasped the concepts as they were being taught, and the numbers used to make my head swim. I taught myself how to write backwards during algebra lessons.

Chris. My best lesson was art and my worst was woodwork. The woodwork master was mad, and taught us all copperplate handwriting when he wasn't rapping us over the knuckles with a length of doweling.