THINK literary bestseller, and something like the newly released Harry Potter And The Deathly Hallows by J.K. Rowling comes to mind - complete with print runs in the millions and spectacular first-day sales.

Then there are books like Kim Edwards' The Memory Keeper's Daughter, which sold modestly when it was published in hardcover in June 2005.

But upon its paperback release in May last year, it skyrocketed to the top of The New York Times bestseller list, and is estimated to have sold 1.3 million copies worldwide.

No one was more surprised by the debut novel's success than the author herself.

'It's wonderful. I guess good reviews spread by word of mouth, and when it came out in paperback, sales kind of exploded,' says Edwards, 49, from her home in Lexington, Kentucky, where she lives with her husband, a University of Kentucky English and linguistics professor, and their two young children.

Unlike more prominent bestsellers like the Harry Potter series or Dan Brown's The Da Vinci Code, there is no magic, thrills, mysteries or epic battles between good and evil in her novel.

There is however, a secret: A doctor hides from his wife the fact that they have a child with Down's syndrome, and this secret slowly breaks their relationship apart.

'People around the world understand the nature of secrets, what it is to keep them and have them kept from us. That is something that really strikes people,' the writer says, when asked to speculate on the book's popularity.

She had published only one book before the novel, a collection of short stories titled The Secrets Of A Fire King (1998), which was critically acclaimed but sold poorly in hardcover.

It was picked up for paperback release only upon the success of her second book.

'A lot of people come up to me after readings and tell me stories of secrets that they have kept during their lives. It is powerful, this idea that we all have turning points where we make a decision, even if it is not as dramatic as the one in the novel,' she adds.

She herself nearly turned down the chance to hear the story that became the seed for the book.

It was 1998, a few months after her first book was published, and one of the pastors of her church took her aside and said she had a story to tell her.

However, it was not the first time someone had offered Edwards a story, and so she declined, feeling that the story 'would not be mine to tell'. Luckily, her pastor was insistent, and a week later told her the story anyway.

'It was about a man who had discovered, late in life, that he had a brother with Down's syndrome who was placed in an institution at birth, and kept a secret from his family,' she says.

Though she realised that the tale would make a good novel, she had her reservations.

'I hesitated to write as I didn't know anything about Down's syndrome - what it is like to have it and what it is like to care for someone who has it,' says Edwards, who received a master's in linguistics from the University of Iowa in 1987, and who taught English in Asia in the late 1980s, including in Malaysia and Cambodia.

'It seemed like an important thing to get right and I didn't know if I could do it.'

But she came to a turning point a few years later, when she was invited to conduct a writing workshop for mentally-challenged adults.

'I was nervous. I had no idea what to expect,' she says. 'But as it turned out, it was a wonderful experience, and some of them wrote very fine poetry.'

Inspired, she decided to research the condition and return to the story her pastor had told her.

'I wrote the first chapter in a month, and once I started, I wanted to know what happens next, and so I took it step by step,' says Edwards, who finished the book three years later.

Even as the novel remains on bestseller lists - it has been on The New York Times paperback list for 56 weeks and is No. 3 this week - she is hard at work on her third book.

Tentatively titled The Dream Master, the novel is set in the Finger Lakes district of upstate New York, where the writer grew up the eldest of four children of a schoolteacher mother and a father who was in the lumber business.

Though fans of The Memory Keeper's Daughter will no doubt snap up the book when it is published, don't expect her to count her chickens before they hatch.

She says: 'I don't know what makes a book a bestseller - I wish I knew so I can do it again.

'But I guess it all comes down to something mysterious that happens between a book and its reader.'

The Memory Keeper's Daughter ($17.12 with GST) is available from Books Kinokuniya.