Gone Are The Leaves, Anne Donovan's unusual new novel, began with a single word.

Of course, all books start with a single word which picks up others as it snowballs from there. But in this instance, it was finding a word in the Scots Thesaurus that set her on her way, as she explains while sipping Earl Grey in an Italian cafe in Glasgow, where she lives with her husband and teenage child. Dressed in powder-blue pea coat and pink pixie boots, with hair the colour of chianti, she looks as if she has just blown in from Italy herself, to cheer this cold, grey Scottish day.

"I came across the word feilamort, the first word in the book," Donovan says, speaking superfast, as is her way. "Feilamort: the colour of a dead leaf. I thought it was a beautiful word. I'm a leaf obsessive. I'm always looking at leaves and taking photos of leaves! I just thought, the colour of a dead leaf - does it have one colour? I wrote down 'Feilamort, the colour of a dead leaf,' and I just started to write. And it was that voice that came to me, and it was a different voice. It was a much older voice, it was not a Glasgow voice at all, it was not a contemporary voice."

As the opening page of the novel reveals, and as Donovan found to her surprise, the reader is back in the middle ages. It is no wonder she sounds shocked. Her books and stories until now have been set in modern times, with characters who speak soft lilting Scots in a west coast accent - as does she. The elfin, slightly shy Donovan was brought up in circumstances that she describes as "very typical of a lot of folk of my generation and the area I came from - normal, Catholic, stable, family was very important and I think it gave me a great deal of stability and support.

"I went to school in Coatbridge. Education was very important too - again, like lots of writers you probably talk to, we got the chance to go to university. Our parents hadn't had that opportunity and were extremely keen for us to do the best we could. But there was a big emphasis from family and school (and church) on achieving, not just for yourself, but because you should use your talents for the good of others, do a socially useful job."

Donovan became an English teacher, not because it was useful but because she had always wanted to teach, and her sympathy with young characters is palpable in all her work. First coming to attention with her short story collection Hieroglyphics, she quickly made a name as a champion of modern Scots dialect. Some were surprised that a novel such as Buddha Da, about a Glasgow painter and decorator who forgoes the pub and takes up Buddhist meditation, was not only warmly received south of the Border, but was shortlisted for the Orange Prize, and translated into German and Russian. (Her second novel Being Emily, she says with astonishment, was translated into German, but also Brazilian Portuguese.)

Those leery of dialect need not worry. The language in the Scots sections of Gone Are The Leaves is not modern, but nor does it feel archaic, and as with all her writing, it reads easily and compellingly, although the writing of it appears to have been somewhat tortuous: "You do wonder, is this ever going to work out?," she remembers.

Ambushed by the word feilamort, and the story that began to unreel thereafter, Donovan had no option but to follow her imagination. Told from various perspectives, although the main character is a young embroiderer, Deirdre, it tells the story of Feilamort, a French orphan and exceptional singer, brought to a wealthy Scottish household, to perform for the lord's wife. In time, it becomes evident that his background is complicated and potentially sinister.

As his relationship with Deirdre blossoms, he reaches the age where to continue to earn his living from his voice a terrible decision awaits. This, it transpires, is the least of the dangers he and Deirdre face in a story which, while in some ways fantastical, is also a closely plotted historical evocation of an unspecified period some time before the tumult of the Reformation. Almost all the characters are strongly religious, and when asked if it was important that the story took place before the coming of Protestantism, Donovan nods emphatically.

"In terms of the changes in the country, although there were changes over centuries, there was not as much as there was post Reformation. Also, if it's post Reformation there's a religious conflict whoever your characters are which means it's necessarily an issue, whereas I wanted it to be more of a given that everybody, whether they are good, bad or indifferent, will hold to the same religious belief system and have the same rituals and things," she explains.

Most writers turning to the past for their story research the politics and manners of the age. Not so Donovan. After getting books out of the university library on embroidery, tapestry, nuns and the like, all of which feature in the novel, she found a richer source in the Scots Thesaurus. This book, she tells readers in a note at the back of the book, is "one of the best reads ever".

She explains: "The way the Scots Thesaurus is organised into different themes, you realise what people were concerned with and what they were like, because some of the words are really specific as to weather conditions, which I think the average person nowadays would never even notice that specifically. Things to do with certain kinds of clouds that presage certain kind of weather conditions, because it was so important what the weather was.

"My main character can't read, so there are no literary references. In my contemporary books there are references to films or a pop star, so there's nothing like that ... You're really looking at what is their life concerned with, and a lot of that came from the thesaurus rather than specific history books."

Among her finds was "mortfundyit", meaning cold as death. Her eyes light up as she recalls another favourite. "'Nirly' with the cold," she says, "it's almost like shrivelled. It's that thing, it's so specific. There's no English word for it." Nor is there for 'flaucht-braid', which means to have your arms stretched out like a bird. "We've lost that," she says, with regret.

Certainly, her mining of the thesaurus for archaic words makes Gone Are The Leaves stand out as very different from most Scottish historical novels, catching a sense of the period, as well as her characters. At its densest, it feels as if one is reading a foreign language, and yet it is completely and rather poetically intelligible. "You really have to get your tongue round it," says Donovan, of this different, more colourful old language.

Apart from grappling with obsolete words, were there any pitfalls she had to avoid, when writing about the past?

"Probably the only worry I would have is that it would be disappointing to people," she replies. "There are people who like well researched and detailed and factually correct fiction, and if they go along thinking that's what they're going to get..." She trails off. "But I think there's no chance they really will, it's not being put forward that way."

For a writer who has drawn so many plaudits for a relatively small body of work, Donovan is surprisingly, and engagingly, self-deprecating. She embarked on this novel when writing another, which was put to one side while she explored where this new book was going.

When finally it was written, she sent it to her agent, telling her it was strange, and that if she didn't like it after a few pages, not to worry. The agent replied, saying she thought the novel "otherworldly", and that they should send it immediately to her publisher Canongate, who snapped it up, much to Donovan's surprise.

"There's a difference," she tells me, "between a book which you had to write, and a book which somebody else has to publish. I've always been quite clear about the difference between those two things. I never think that because I've been published in the past, I'll necessarily..." Again, the sentence goes unfinished. "I'm not being falsely modest," she continues. "I was actually shocked that they took it in the first place, I just thought it was so mad."

Gone Are The Leaves is published by Canongate, £12.99. Anne Donovan is speaking at Looking Glass Books in Edinburgh on May 8 at 6.30pm