Like first loves, every writer has a special relationship with their first book. I look back at The Poison Tree now with a mixture of fondness and regret. It contains some of the best writing I've done, and probably some of the worst too. It's become so much a part of me that bits of it keep cropping up, by accident, in other books I write.

The idea came from two short stories I wrote which somehow got stirred together and started fermenting. There was one about a woman who moves into a new house and discovers a cat, left behind by a previous owner, which promptly gives birth and over the next few days starts to eat its own babies as the consequence of some mysterious trauma that happened in the past, until the next door neighbour offers to drown the survivors in a bowl of water. Then there was another story about someone who is doing up a new house and finds a stack of pornography. I wrote the whole book in about four weeks, got to the end, and discovered it was only sixty pages long. So - not a novelist after all, Tony, I decided. Then a year later I re-read it and realised that I'd just compressed what should have been 90,000 words into 30,000. I simply had to go through and unpack it - the literary equivalent of clicking on a Winzip icon - and lo, I had a book.

I had a friend who worked for a publishers, who gave me a couple of agent's names: both agents called me in for a chat, so I went back to my friend for advice on which to choose. She gave the manuscript to an editor, who actually made me an offer straight away. Sounds like a dream, particularly when the reviews turned out to be pretty good as well, but actually there isn't a huge market for the kind of stuff I write and I've always had to choose between writing what I regard as interesting original stories and what the market regards as saleable - ie stuff that simply reclothes a formula. There are a lot of good writers out there but there are also a lot who are pretty interchangeable.

I still can't quite believe I put so many explicit sex scenes into The Poison Tree. And that I had the nerve to create a heroine who's a bolshie, arrogant lesbian - and then to show her, graphically, in bed with her girlfriend. Not surprisingly, some people thought that was prurient on my part. (Though, interestingly, some people had assumed from the start that I was a woman, and those people didn't mind the sex so much). I did the explicit stuff because I figured that since my killer was a sexually motivated killer, it would be neat to juxtapose his twisted, negative view of sex with my protagonist's more personal sexual history. Incidentally, I agonised over whether to 'show' my readers the pornography she finds, or simply to refer to it: in the end, I decided that it was more prurient to withhold it. I had been reading a lot of Nancy Friday, which probably made me more interested in the subject than I should have been. There are several scenes that, if I were writing the book today, I wouldn't include. Kill your darlings! On the other hand, I definitely fell in love with Terry as I wrote her, so that maybe excuses some of it.