Nick Hornby says the subject is "ripe for a kind of black humour"

In the book, four people meet at the top of a tower block after independently deciding to kill themselves by jumping off.

When each discovers why the other is there, they decide not to go through with it - and the book follows what happens to the group afterwards.

The idea came from statistics showing suicide rates spike on certain nights of the year, such as New Year's Eve and Valentine's Day, Hornby told the BBC World Service's The Ticket programme.

He then read about someone who killed themselves at Beachy Head, a notorious English suicide spot.

"I realised that at somewhere like Beachy Head, people are bound to go and actually not jump," he says.

My tendency is to make jokes about grim situations, sometimes to the irritation of my loved ones Nick Hornby

"If you saw someone else at one o'clock in the morning at one of those points, you'd pretty much know what they're there for.

"I just started to wonder what sort of relationship could develop between people like that."

Hornby changed the location so his characters could not escape from each other at the beginning - at the top of a tower block, "they have nowhere to go."

This also gave him the chance to create a group of four very different characters, including a failed musician and a washed up TV presenter, who would not normally meet.

Hornby developed the novel after reading about Beachy Head

"I think as a writer you spend a lot of time looking for situations where people move out of their own age and class to talk to people," Hornby says.

"It's hard to do in life, and it's a gift if you can think of a situation in which that's likely to happen."

In the book, he wanted to take extremely depressed characters and "move them towards the light" without being sentimental and unrealistic, he says.

Hornby feels the book is "essentially optimistic" and the author liked the idea of writing a positive novel about depressed people.

There is also much dark humour - for example, the group form a book club that only discusses authors who write about suicide.

"I can't imagine writing anything that didn't have jokes in it, and I thought that it was a situation that was ripe for a kind of black humour," he says.

Hornby (back right) gave extracts for the tsunami New Beginnings project

"I don't find it difficult to write jokes about those situations - it just feels like a natural extension of personality.

"My tendency is to make jokes about grim situations, sometimes to the irritation of my loved ones."

But he also stresses A Long Way Down is not a comedy and says it tackles suicide in a serious way.

One character does kill himself halfway through. "It is a serious business, and lots of people have lost people," Hornby says.

"I didn't want to give the impression that all you've got to do is talk a bit, and then it's all over... [the character who dies] has made his mind up, and there's nothing anybody can do.