DIANA Gabaldon’s 10th Outlander book will be the final in the series … maybe. She isn’t sure. She always thinks it’s the end and then it’s not.

Since the late 1980s, the American author has written nine best-selling books that have proved to be a sensation across the globe.

It started as practice writing not intended for publication, penned in the early hours while her husband slept, before heading off to two jobs a few hours later as an assistant professor and a software reviewer.

But now her late-night stories have branched out a little bit.

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Averaging around 900 pages per publication and a family tree fit for a forest, each book can take years to make, with her 2021 release having been in the pipeline since 2014.

Gabaldon told The National that while she came up with the ending 20 years ago, she still isn’t quite sure whether the 10th book will mark the end of the series and see the dreaded droughtlander continue in perpetuity.

Asked if number 10 will be the last fans will hear of Jamie and Claire, Gabaldon said: “I don't know. I thought Outlander was going to be a practice book that no one would ever see and here we are.

“Number 10 is the last one I think, it’s the last one I intended. But things happen so I don’t know.

The National: Diana Gabaldon was in Edinburgh to promote her ninth Outlander book, and visit her daughterDiana Gabaldon was in Edinburgh to promote her ninth Outlander book, and visit her daughter

“On the other hand, I’m 70 years old so I don’t know how long I’m gonna last either. It might be the last one whether I want it or not.

“Although luckily I come from a very long-lived family in terms of the women. They all live well into their 90s.”

Despite not knowing whether the current book will draw a close to the 30-year saga, the ending has been known – to Gabaldon and a select few others – for more than 20 years now.

“I got up as everyone else was asleep, and just got a notebook out of my bag, sat on the couch and wrote it down with tears running down my face,” she said.

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That was two decades ago while visiting her in-laws. But it doesn’t quite mean the end of the Outlander universe.

In more recent news, Starz (which make the show) announced a prequel is in development, and Gabaldon confirmed she too is writing the book that will lay the foundations for the TV series.

The author also confirmed she would be involved in the making of the TV programme, for which she supplied a three-page working synopsis.

The National: Diana Gabaldon is working on a prequel to the hit Outlander seriesDiana Gabaldon is working on a prequel to the hit Outlander series

She said: “A prequel has been on my mind for some time. It's Brian and Ellen's story, Jamie's parents. I know the basics of their story. In fact, Jamie told it to Claire in the first book, so it's always been there.

“As I return to Scotland and do more and more research, you pick up a lot of things that aren't actually relevant to what you're doing, but may come in handy.

“I walked the battlefield of Sheriffmuir, for instance. It was probably the most indecisive battle ever fought. Essentially, both sides just gave up and walked away. No one's ever been able to tell me why.

“There is a whole lot of history going on there and I thought it would be interesting to explore Brian and Ellen’s story more because perhaps they were involved with the earlier parts of the Jacobites.”

The National: Diana Gabaldon said a prequel is in the worksDiana Gabaldon said a prequel is in the works

When Gabaldon wrote the first Outlander book, originally published under the title Cross Stitch in the UK, she had never been to Scotland.

Midway through the second though, she decided to take a few weeks to take in the country in which her stories were based – and since regularly comes back.

Gabaldon used her time in Scotland well in her first trip in 1992, where she picked up enough books to fill a car trunk.

“Every bookshop I went into, I'd come out with a handful of books. We had to take several suitcases back with us to supply this new research library.

“But we loved it. We loved the people, and the music, and I love the climate, which is very much unlike Arizona.”

When Gabaldon comes to Scotland, she says there are several places she frequents, but there’s one place she always comes back to.

“I’ve been very fond of Inverness. It actually reminds me of the place where I grew up in in Flagstaff, Arizona.

“It's in mountains and it's got a generally cool temperature, and it's got that nice, smallish town feel. Though like Flagstaff, it's also expanded somewhat of latter years.”

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Outlander has become such a phenomenon, Gabaldon often hears from fans touched by the series. One fan who wrote to her thanks the author for getting her through a particularly rough time in her life.

“One woman wrote to me and said ‘I’ve always loved your books and admired the strength of your characters, especially Claire’,” Gabaldon said as she began to tear up.

“Her brother-in-law, at one point had shot himself in his wife's living room, very traumatic, of course, to the family, and to her sister.

“She said she'd seen her sister safely taken care of, and then went back to the house. And she said, ‘Well, I just said to myself that Claire can do the thing she did I could do this’.”