Like all great ideas, it is so blindingly obvious that it is astonishing nobody had thought of it before: a detective novel set in Shetland.

As a location, it's perfect - an enclosed community, a dramatic landscape, incomers versus the established community, a summer season of tourists to add new flavours to the mix. It is the traditional village mystery transplanted to the North Sea, with the remnants of the oil industry and vast trawlers as a backdrop.

Ann Cleeves had already experienced Shetland life - she was the cook at a bird colony on Fair Isle for two years in the 1970s - and been writing detective fiction for nearly 20 when she hit on the idea for Raven Black. A coot had been spotted, a rare sighting in Shetland, and she took her husband, Tim, a dedicated bird watcher, on a one-day trip to see it.

“It was between Christmas and New Year, when it's dark most of the day. Finally, the sun came up. It had snowed, then frozen on top, and there was ice on the shore. We saw three ravens, black against the snow. It was such a striking image I thought, ‘If there were blood as well, it would be like a fairy story, Snow White with her blood-red lips and all that.'

“I asked Tim, ‘If there were a dead body, would they feed off that?' And he said. ‘Of course,' so I had that first scene, and I had to start.”

If Cleeves were a rock band rather than a writer, Raven Black, published in 2006, would be her breakthrough record. It uses the tensions of island life - the impossibility of keeping secrets, the juxtaposition of broadband and Up Helly Ah - to frame characters that are human, believable and compellingly flawed.

Add in the rivalry between the local police officer, Fair Isle man Jimmy Perez, and the arriviste from Inverness, to say nothing of the intriguing single mother Fran Hunter, and you have a compelling 320 pages.

Cleeves, who had been what publishers call “a mid-list author” - “there is no bottom list, just the mid” - for many years, submitted the manuscript to Pan Macmillan with no great expectations. “It's easy to get lost in a big publishing house that publishes Colin Dexter unless you're selling relatively well, ” she says.

Happily for Cleeves, it arrived on the desk of a new young editor who loved the book and was determined to spread the word. “She admitted she didn't have much of a marketing budget, but she really pushed it, sent sample chapters out to all the reps and submitted it for an award,” says Cleeves, who is still readjusting to life beyond the mid-list. “After 20 years, I was suddenly touring the United States, going to Japan, and my French editor sent me tickets for Paris.”

The editor's faith was vindicated when Raven Black won the Duncan Lawrie Dagger, an award for crime fiction. Nobody was more astonished than Cleeves, who still can't believe she is reviewed, flatteringly, in the Times Literary Supplement and is posted packages of translated novels.

These are now joined on the shelves by White Nights, the second book in her Shetland series. She has just submitted the manuscript of the third. Each one is set in a different season: White Nights refers to the endless summer days, when it is never truly dark and the island is sleepless.

Cleeves has gone from being convinced there was only one murder for Perez to investigate to being persuaded that four would be just about believable.

“There is a limit to how many murders you can realistically have in a place like Shetland,” she says. “On the other hand, it is such a rich location, it will be a wrench to leave. If you write traditional crime, it's like Agatha Christie's ship on the Nile, or the train stuck in the snow.”

The book has been welcomed by everyone in the tourist industry. They hope it will bring visitors to see the long summer days. And instead of kicking themselves for not writing these books themselves, local writers have been generous

to a self-proclaimed outsider.

“One said to me, ‘I'd never have been able to write these books.' One of my characters is called Robert Isbister. He knows seven Robert Isbisters and they would think the book was about them.

“I had to be an outsider to write these stories. A local would have been so worried about offending people. I've got friends there, but it's not the same. That was never an issue.”

A grandmother in her fifties, Cleeves has a soft northern accent and a self-deprecating manner. With her short hair and enamel earrings, she looks like the social worker she once was. In fact, she began writing once she had given up meeting parole boards and found herself stuck on a tiny tidal island with only an infant and many birds for company. By this time, Tim was warden of Hilbre, an 11-acre tidal island between the Wirral and north Wales. “We had no running water and no mains electricity,” she says. “I would walk out across the Wirral in the morning in my wellies and oilskin. The council gave me a hut on West Kirby prom, where I kept my other clothes. I would get changed, then get the train to work in Birkenhead.

“When I got pregnant I gave it up with a sigh of relief, but then I was stuck on the island with not much else to do - if you're a bird-watcher, it's fine.” She leaves a significant silence.

Cleeves loves Tim very much, but she is certainly not a twitcher. “In fact, my first book was about bird-watchers - someone gets hit over the head with a big brass telescope.”

Having devoured Agatha Christie as a child, before moving on to

Dorothy L Sayers and Margery Allingham, Cleeves produced her first books without realising that the genre had moved on since the 1930s.

“When I started writing, I thought crime fiction had to have a very upper-class detective, because Dorothy L Sayers did. So my first detective had a double-barrelled name - George Palmer-Jones - and had been to a posh school,” she says.

“This is nothing like my background at all, but I thought that was what crime fiction was.”

It wasn't until she started reading the work of living writers that she realised her own experiences provided far richer material. I'd worked as a probation officer in Merseyside, going into people's houses and having the licence to ask them all these questions like, ‘Why do you think your son started nicking cars?' ”

These early books are out of print, much to her relief. “They were pretty awful,” she says. “I see them occasionally on library shelves and I wonder if I could sneak them out.”

Since Palmer-Jones hung up his binoculars, she has concentrated on two characters based in Northumberland: Detective Stephen Ramsay and Inspector Vera Stanhope. Vera seems particularly close to the author's heart, the only fictional cop that, to her knowledge, is overweight, middle-aged, gets out of breath, has no man in her life and could no more run across a rooftop and leap into a murder scene than date George Clooney.

Such are the pressures of leaving the mid-list that Cleeves is having to give Vera an extended holiday, at least until the Shetland quartet is

finished. The third novel is set on Whalsay, the richest of the islands.

“The pelagic (deep sea) fleet is based there,” she says. “There's a lot of money around. The schools are the best-equipped I've seen, the community halls amazing. It makes it very interesting to write about.

“When I first went, it was the start of the oil boom and Lerwick had the feel of a gold-rush town. Now people are used to a certain standard of living, and it's interesting to chart that change. The oil is on the way out, but with fishing, people can still make money. It's a very different world.”

White Nights by Ann Cleeves is published by Pan Macmillan, £12.99