Harkness insists her story is very much its own beast, something that will become clear after reading Shadow Of Night, which finds the two lovers journeying back to 1590. "It was fun for me as a historian to really think about how challenging it would have been for them to do that," she says. "I looked back at my own experiences, learning about various aspects of the period that I didn't already know much about, such as transportation. That really informed my writing of what Diana's experience would be as a historian to go back in time."
Particularly enjoyable was the chance to just make stuff up. "I had enormous fun filling in the historical blanks, which is something I can do with fiction that I can't do when I'm a historian," says Harkness. "If there's a historical blank, I have to leave it blank but as a fiction writer I get to fill it in. Unlike Diana, I'm actually a historian of 16th century London, so I was very much on familiar ground with School Of Night and hopefully I can introduce a wider circle of readers to how special the city of London was during that late-Elizabethan period."
To that effect, the novel features cameo appearances from a host of faces from the era including Elizabeth I herself, William Shakespeare and Christopher Marlowe, all real-life acquaintances of the little-known Elizabethan poet Matthew Roydon, who plays a significant part in the unfolding plot.
"I used his life as a template so I knew if he was somewhere, then Matthew and Diana were there too," says Harkness. "So the known historical contours of Matthew Roydon's life helped to shape what Diana and Matthew's experiences would be in the past. We don't know much about Matthew Roydon, which is frustrating as a historian but as a novelist it provides plenty of wonderful opportunities for invention."
Having recently started work on the third and final volume, Harkness refuses to reveal whether Matthew and Diana will return to the present day.
"People are going to have to wait and see to find out how we wrap up the story and this struggle between witches, vampires and demons over what rights they should have when it comes to things like marriage, reproduction, secrecy and openness."
But she does promise to resolve the deep divisions surrounding Matthew and Diana's relationship, which is frowned upon by both their kinds.
"I've always said that the first book was about falling in love, which we all do very quickly, and just like all of us they went through the giddy stage," she says. "So if Shadow Of Night is about trying to stay in love, the third book will be about making those long-term adjustments that allow you to settle down into a long and hopefully rewarding life together."
Like Twilight and Harry Potter, the All Souls Trilogy could soon be making the leap to the big screen after being optioned by Warners, which has hired screenwriter David Auburn (Proof, The Lake House) to adapt A Discovery Of Witches.
"It's amazing that somebody would want to take your words and transform them into moving images," says Harkness. "Whenever I think of them bringing those scenes in the Bodleian to life, my breath just catches."
Shadow Of Night (Headline $36.99) is out now.