"I owe it all to a Scotsman I met by accident in New York City. With instant faith I explained to him that once every year I go home to Britain to spend a holiday with my oldest friend Zoe, and that this time we'd chosen the Highlands of Scotland - but didn't know where to go. Could he recommend a good hotel?

"The Airds Hotel," he said instantly. "Port Appin, Argyllshire. You'll not beat it anywhere."

He was a very impressive Scotsman, with a beautiful accent. So some months later Zoe and I and Zoe's car took an overnight train from London to Fort William, Argyll. In pouring rain the next morning, we drove down the west coast to Port Appin, below cloud-wrapped hillsides, along misty grey lochs. And out in the rainwashed water of Loch Linnhe, as we rounded a bend toward the hotel, we saw a very small island, not much more than a rock, on which stood a square solitary castle.

Zoe said later, "I remember when you first saw the castle. You were very quiet for a while, and I knew something was going on in your head."

What I could hear, in my head, was a small delighted voice crying, "That's where the Boggart lives!"

In the past 15 years I'd written only one fantasy novel. Finishing the last volume of 'The Dark Is Rising' sequence in 1977 had left me drained of ideas for books, and I found myself writing for the theatre, film and television instead. Apart from 'Seaward', published in 1982, my only works on the children's list had been texts for picture books.

But in that indistinct part of the mind where the imagination lives, I'd begun in the last year or so to find that I wanted to write a book about a boggart, the mischievous, invisible house spirit who is found in the folk-tales of Britain - particularly the North of England and Scotland. Why a boggart? I don't know - I just liked the idea of boggarts, their quirkiness, their independence, their mischief-without-evil. And I knew I wanted to bring my Boggart - once he becomes particular, I give him a capital letter - accidentally across the Atlantic, to find out what would happen to him. Perhaps I thought that since I'd lived in the United States for 30 years, it was high time I set a novel there.

Zoe and I had a splendid holiday at the Airds Hotel, and I went often to stare at the castle - always from across the water, since it was unoccupied and locked, and no one was allowed inside. Then I went home and started to write 'The Boggart'.

For me, writing a book is always a wonderful voyage of discovery, as the story unfolds itself and the characters appear. It never fails to surprise me, probably because I am the kind of author who starts a novel knowing a lot about the beginning and the end and hardly anything about the middle. The largest surprise to me this time was the place to which my Boggart's transatlantic voyage would take him. I'd assumed it would be Boston, the American city I know best. But my imagination wouldn't give me any pictures of Boston. It kept showing me Toronto.

I'm very fond of Canada; perhaps I see it as a halfway house between Britain and the United States. I give all my manuscripts to the Lillian H. Smith Collection in the Toronto Public Library, I visit the city often, I have friends there - "But I'm not Canadian!" I said, to the voice in my head.

"But you're not Scottish either," said the voice.

"So?"

So The Boggart is set in Port Appin and in Toronto, not in the United States. And the next book won't be American either, because last summer Zoe and I went back to Scotland, to the Isle of Skye, in the Hebrides. To get there we drove across Scotland from Inverness (after another overnight train ride), all along the length of Loch Ness, and I looked out at the dark rippled water of that magical lake and found the Boggart inside my head again, trying to get out.

As a result, before long I shall be writing one more book about him, set entirely in Scotland, and I hope I shall have as much fun as I did writing the first. And perhaps one of these days my head will give me a book set in New Mexico, or Seattle, or even Boston - but I can't make it happen. I just have to listen for the instructions, and do what I'm told. You can't argue with that small voice of the imagination."