Does anyone actually know the answer to this? I suspect not – if they did, they’d all be doing it and we’d be awash with bestsellers.

But I’m asking the question because I’m off to the Tamar Valley Writers Festival this week, and this is the title of a panel I’m on. When I first saw it, I blinked and thought, what on earth am I going to say? I’ve got a bit more of an idea about it now, but mostly it boils down to ‘no one knows’.

Some people think there’s a formula, but they’re usually people who have never actually written anything. Some people think it’s up to the publishers – if they push the book hard enough, it’ll become a bestseller. (It’s often disgruntled authors who think this. ‘If only they’d pushed my book harder …’) But there are any number of stories about publishers who have thrown their weight (and their money) behind a book only to have it die a lonely death.

There’s a lot of luck involved, in my opinion. If your book was due to come out on September 12, 2001, you didn’t have a hope, no matter how wonderful it was. And you might be writing about something that is really topical, only to have the world change around you, so that by the time your book comes out, no one is interested.

A successful book captures the zeitgeist – the spirit of the times – in some way. There were quite a few marvellous books about kids and magic before Harry Potter. But somehow, JK Rowling hit exactly the right moment, with exactly the right story.

I suspect that some of the bestsellers from twenty years ago would hardly make a mark now. Mightn’t even get past the slush pile. And some of today’s bestsellers would have been thrown out the door if they were submitted back then.

I’m not saying this is always the case – there are probably books that will always capture an audience. But fashions change, and books change with them, thank heavens. E.g. Charles Kingsley’s ‘The Water Babies’, which was once a popular children’s story, and now seems pompous and didactic (though I love that cover).