On Halloween morning, the shed in Joanne Harris's garden had transformed into a yellow pumpkin whose colour darkened to burnt-orange throughout the course of the day and lit up inside at night.

The following day it had turned into a carnival hall of mirrors, all of which showed her as short and squat.

The day before it had been a small, high window with a restricted view of pigs on the wing.

This is, at least, how she described it to her 4,600-plus Twitter followers - the word "shed" always beginning with a capital letter as if it were an animate being.

It is inside this small stone construction, with its reclaimed slate roof and oak-framed windows, that Harris disappears into the different worlds of her imagination.

At the moment she is making regular visits to France to find out what Vianne Rocher and her daughter Anouk are up for the third novel in a series that began with Chocolat, later turned into a film starring Juliette Binoche and Johnny Depp.

Recently, however, the bestselling author has found herself transported to her own version of the Nine Worlds which, according to Norse mythology, sit in the branches of the World Tree, Yggdrasil.

On Friday, Harris will hold an "audience with" event at The Civic, in Crosby, to mark the publication of Runelight, the second book set in the Norse pantheon.

Her take on the Nine Worlds has a second origin - in a story called Witchlight that she wrote as a teenager, which starred a young girl called Maddy.

The daughter of Thor the thunder god, she survives as the central character in Harris's series.

"That particular type of mythology is very accessible to children because, unlike the way the Greeks and the Romans are presented, which tends to be quite remote, there is something about Norse gods that makes them very human," says Harris, who has enjoyed reading mythology since she was a child.

"A lot of them are not particularly admirable humans either. There's a lot of meanness, stupidity and mischief. They have an awful lot of human failings which makes them fun."

Harris began writing Runemarks, the first book set in this universe, when her own daughter, then aged eight, wanted to find out more about the characters in Witchlight.

At the time she had not considered whether to show the book to a publisher.

"We had a great time," she recalls.

"I read it to her chapter by chapter as it was finished and in some ways she shaped what was going on because it became clear who she liked and who she wanted more of."

As well as Maddy, who grew up among a non-magic family with no idea of her powers or god-status, Harris's daughter was fond of the mischievous trickster, Loki, who consequently became a main character. Runelight takes place three years later, when Maddy and the handful of gods who escaped from Hel in the first book have a new challenge to overcome and a potential new enemy - Maddy's sister.

Harris, who is regularly sent - and promotes - stories based on her own writing, sees her take on Norse mythology as a sort of fan fiction.

"I think fan fiction is a very natural thing for people to do, particularly for kids," she says. "It's about embracing characters and a universe of someone else's creation and making them your own.

"What's available about the Norse gods in writing is so limited you're basically looking at scraps of stories that are being pieced together.

"I wanted to know more."