It is two in the morning in the Arizona desert and lightening is flickering across the sky.

There has just been a rain storm, and another may be on its way. Toads are singing under the mesquite trees, and everyone is asleep. Everyone, that is, but novelist Diana Gabaldon, who is in her study, a candle burning on her bookshelves as she talks to me by phone and describes the scent of the desert which, she says, anyone from Scotland would find "overpowering". Keeping her company are two dachshunds - "ideal writers' dogs because their idea of bliss is to curl up on your feet and go to sleep."

It may be an ungodly hour, but the middle of the night is when Gabaldon is at her best. For over 20 years, the internationally bestselling author of the Outlander series of fantasy historical novels has packed her husband Doug off to bed at half past nine, kipped for a few hours on the sofa, then woken at midnight and gone to her desk, where she works until half past four in the morning.

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This unorthodox regime began as a convenient way of splitting responsibilities with Doug, who ran a computer programming business, when their three children were young. "I would take the evening watch and be available to fetch them from dates or take them to the prom or stand by for midnight crises, or whatever, and my husband would do the morning stuff as I'm not good for anything in the morning," she says, in her deep, husky voice.

Now the children are grown, but the habit persists. At the hour we're conversing Gabaldon would normally be immersed in the rococo twists and turns of her latest book, but she has just finished a punishing promotional tour of Canada and the US for Written In My Own Heart's Blood, the ninth in the series, and has not yet got back into the routine.

Gabaldon's novels are as original and unusual as their creator. Initially set in the Jacobite era, though now creeping into the later 18th century, they jump between past, present and the mid 20th century. At their heart is the romance between a young Jacobite, Jamie Fraser, and Claire Beecham, an English battlefield nurse who, when she finds herself in a circle of standing stones in the Scottish Highlands, is transported back to 1743, the age of tartan plaids, broadswords and Bonnie Prince Charlie himself.

So much happens in the series that one would need nanotechnology to condense the action into a soundbite. Written In My Own Heart's Blood, for instance, set during the American Revolutionary War, opens with the newly widowed Claire discovering her dead Jacobite husband has returned from the grave, while her modern-day husband has gone back to the perilous past. That outline, though, barely begins to hint at the complexity of what follows.

Suffice to say that Gabaldon's writing moves at a missile pace, much as its author talks, and is distinguished by the dry wit that colours her conversation. Purists such as myself may be shocked at the licence Gabaldon takes with history and reality. She boasts, however, that she has promised a dollar to any reader who, after sampling three pages, can put the book down. To date, it seems, nobody has claimed their reward.

Regardless of the audacity of these books, what is not in doubt is the panache with which her stories unfold, nor the vivacity of her characters and her skill at setting a scene.

Such storytelling talent is rare in a former university professor. Sixty-two year-old Gabaldon, who was born in Arizona, has degrees in zoology, marine biology and a PhD in behavioural ecology. Outlander, her first novel, was written as a private experiment in her mid thirties. This was no idle dabbling, however. From the age of eight, says Gabaldon, "I knew that I was meant to be a novelist, I just didn't know how one does. I came from a very conservative background. My father was fond of saying to me, 'Well, you're such a poor judge of character, you're bound to marry some bum, so be sure to get a good education so you can support your children.'"

Gabaldon and her only sibling followed that advice, her sister now holding an endowed chair on the law faculty of George Washington University. Neither, clearly, is afraid of hard work. Since Outlander was first published in 1991, Gabaldon has published around 20 books, and within a month of publication of her newest title has signed more than 38,000 copies. Such drive, she tells me, can be directly attributed to her upbringing. "My father was the 13th and last child of a very poor New Mexican dirt farmer who died three months after he was born, so he grew up in dire poverty. The work ethic is very firmly embedded in his family."

Gabaldon began writing her first novel in secret. That it was set in Scotland is remarkable, given she had never visited the country. Was it the contrast between the hot, dry desert and cold, wet Scotland that sparked her imagination?

She chuckles. It was nothing so obvious. Wondering what would be the easiest genre for practice, she settled on historical fiction. "As a research scientist I knew my way around a library and it's easier to look things up than to make them up. I was casting round for a convenient time and place, and I happened to see an old Doctor Who." In this episode, the Doctor had picked up a young Scotsman, from 1745, who appeared in his kilt. "I thought, that's rather fetching, and I was still thinking about this the next day. So I thought fine, Scotland, 18th century."

Plunging in, Gabaldon researched on the hoof. Her writing technique - which remains the same today - would give many less gung-ho novelists goosepimples. "About the third day of writing I introduced this Englishwoman. I had no idea who she was, or what she was doing there, but I introduced her into a cottage full of Scotsmen, just to see what she did. Anyway, she walked in and they all turned round and stared at her, and I thought why? Was it that unusual to see an Englishwoman? Actually it was. In the 18th century, nobody went to the Highlands unless they were in the army and had to, but I didn't know that at the time.

"And one of them drew himself up and said, 'Well, my name's Dougal Mackenzie, and who might you be?' And without my stopping to think I typed, 'My name is Claire Elizabeth Beecham, and who the hell are you?' And I thought, you don't sound at all like a historical person, so I fought with her for several pages trying to beat her into shape and make her talk like an 18th-century woman. She wasn't having any of it. She just kept making smart-ass modern remarks."

At this point time travel was introduced, a device Gabaldon did not worry about initially, since she had not planned to show anybody the book. When eventually she did, its enthusiastic reception astonished her, as did the three-book deal that followed.

Jamie Fraser quickly became one of the most dashing and likable romantic heroes in recent fiction. He is soon to garner a whole new audience now that the televised series of Outlander has started in America this weekend. Already it is attracting warm reviews, its spectacular Scottish setting in particular singled out for praise. Sam Heughan, who plays Fraser, is well cast, and Gabaldon is delighted with his portrayal. Understandably protective of her fictional character, when asked if he was based on anyone she knew, she sounds amused.

"Like most characters he is, if not a reflection of the author, at least a refraction of part of my mind and background and character. He is for instance very tall with red hair because my husband is very tall with red hair, because that's the sort of man I find attractive."

When Doug first read the book, he noticed the similarity. "I said it's true, you're Jamie Fraser's body model from the neck down."

A practising Catholic, Gabaldon admits that she has occasional flashes of insight from what she calls "the other side", as when she hears dialogue being spoken in her head. Whether her sense of feeling at home in Scotland when she finally visited was due to her excellent research skills, or "spiritual coincidence", she is still not sure.

"From the moment I began researching Scotland, I did feel a fairly strong affinity for it, and having once been here, I was committed and I come back at every opportunity."

Her most recent trip was in February, for a small cameo part in the TV series. "Well, it's indentured servitude! They work 12 hours a day, you are compelled to come in at dawn in order to be made up and costumed, and you're passed from hand to hand all over the set. You're always escorted by a minion with a walkie-talkie, and it all runs to a very strict timetable, so they make sure you are where you're supposed to be when you're supposed to be, wearing what you're supposed to be."

She sounds mildly affronted. "It's like being treated like a parcel." Gruelling though it was, one suspects that for Gabaldon, compared with the day job, it must have felt like a holiday.

By the time our conversation draws to a close, dawn will soon be breaking in Arizona, and I leave Gabaldon in her candlelit study, surrounded by hundreds of books. It's safe to say that few will be as idiosyncratic as hers, and even fewer have as many fans.

Written In My Own Heart's Blood is published by Orion, £18.99. Diana Gabaldon is at the Edinburgh International Book Festival on August 18