ORE THAN TEN YEARS ago I started to write a children’s book. It was for my daughter, Holly, who was five years old. I wanted it to have a girl as a heroine, and I wanted it to be refreshingly creepy.

When I was a boy I lived in a house that had been made when a larger house had been divided up. The irregular shape of the house meant that one door of the house opened onto a stark brick wall. I would open it from time to time, always suspicious that one day the brick wall would be gone, and a corridor would be there instead.

I started to write a story about a girl named Coraline. I thought that the story would be five or ten pages long. The story itself had other plans.

We moved to America. The story, which I had been writing in my own time, between things that people were waiting for, ground to a halt.

Years passed. One day I looked up and noticed that Holly was now in her teens, and her younger sister, Maddy, was the same age Holly had been when I had started it for her. I sent the story so far to Jennifer Hershey, my editor at HarperCollins. She read it. “I love it,” she said. “What happens next?”

I suggested she give me a contract, and we would both find out. She agreed enthusiastically.

I bought a notebook, and started to write in it. It sat on my bedside table, and for the next couple of years I would scrawl fifty words, sometimes a hundred words, every night, before I went to sleep. A three-day train journey across America was an opportunity to work, uninterrupted, on Coraline. Getting stuck on American Gods, a long novel I was working on, gave me the opportunity I needed to finish Coraline’s story. A year later I wrote a chapter I had meant to write but had never got around to, and Coraline was finished.

Where it all came from—the other mother with her button eyes, the rats, the hand, the sad voices of the ghost-children—I have no real idea. It built itself and told itself, a word at a time.

A decade before, I had begun to write the story of Coraline, who was small for her age, and would find herself in darkest danger. By the time I finished writing, Coraline had seen what lay behind mirrors, and had a close call with a bad hand, and had come face-to-face with her other mother; she had rescued her true parents from a fate worse than death and triumphed against overwhelming odds.

It was a story, I learned when people began to read it, that children experienced as an adventure, but which gave adults nightmares. It’s the strangest book I’ve written, it took the longest time to write, and it’s the book I’m proudest of.