While researching his best-selling mystery novels, Irish writer John Connolly has visited some creepy places. He has been to supermax prisons and walked among the Wittenham Clumps, a pair of wooded chalk hills in rural Britain where Iron Age remains of human sacrifice were found.

But, he says ruefully, he slept through a visitation from his late father, who died of cancer while Connolly was in his early 20s.

"He was a terrible snorer," he recalls. The night before the funeral, he says, all the relatives in the house were awakened by loud snoring - except for Connolly, who slept quietly through it all. "Everyone else stood in the hallway listening to my dead father snore."

The 51-year-old stopped over in Singapore last Friday, on his way to Australia. He has published more than 20 books, including A Book Of Bones, out earlier this year.

It has been 20 years since he made his debut in 1999 with Every Dead Thing, featuring Charlie Parker, a detective with a dark past who hunts occult evil-doers.

He wrote his first novel while working night shifts as a reporter at The Irish Times. This was possible, he says, because the editors rarely wanted to change the pages so late unless huge news broke. "I would just check the wires, make sure the Pope hadn't died."

Early drafts of Every Dead Thing met with rejections. When it was picked up in a six-figure book deal by major publishing house Hodder & Stoughton, it was a shock to everyone, including one of Connolly's former editors, who reportedly had to be helped to a chair, wailing: "But he wasn't even a good journalist."

Connolly, who has since sold more than 15 million books worldwide, thinks the resistance to the first novel was due to its odd structure - "like an hourglass" - its offbeat characters, including a hitman and a thief who are a gay couple, and the way it blithely mixed the genres of crime and the supernatural.

"I didn't think of it as colouring outside the lines," he says, "but there are people who don't think crime fiction should have ghosts in it, which seems to me a dull way of looking at the world."

He likes the South-east Asian approach towards the supernatural, which he feels resembles the Irish one in how matter-of-fact it is.

"There is a similar outlook towards folklore and mythology, the idea of a spiritual world co-existing with the physical one without colliding."

He says he may set a future book in this part of the world, having just finished a novel set in Arkansas.

In A Book Of Bones, the 17th Parker novel, his protagonist goes to Europe on the trail of a murderous immortal, Quayle, and an unspeakably evil book, the Fractured Atlas, even as the bodies of young women begin appearing in places of ancient dark magic, such as a church on the moors or a Saxon mound. It is nearly 700 pages long, although Connolly maintains that 95 per cent of his research does not even make it into his books.

"Every book I've written, I've wanted to throw it away at 20,000 words in," he says.

Besides the Parker series, he has written other books such as The Book Of Lost Things (2006), about a boy struggling with his mother's death who is transported into a fantasy world, and space-war series The Chronicles Of The Invaders (2013 to 2016) with Jennifer Ridyard, his partner of 17 years to whom he recently got engaged.

But he never leaves a work incomplete, however difficult he may find it. "An unfinished thing has no place in the world. You're born with a limited amount of creative confidence. Every time you abandon a project, you chip away at that confidence until all you're left with is fragments."

He thinks of his books as ultimately hopeful, despite their grisly contents and the recurring theme of grief and loss.

"The writer William Gaddis once said, 'You get justice in the next world, in this world, you have the law.' Crime novels, at least the ones I write, don't accept that we should have to wait until the next world. In the Parker novels, people refuse to stand by. I couldn't write a book in which no one makes a difference."

• A Book Of Bones ($29.95) is available from Books Kinokuniya.