The pair find themselves spending time with Matthew's friends, who are known as the School of Night.
This group includes the English poet Christopher Marlowe, who died in suspicious circumstances, and George Chapman - not the infamous Jack the Ripper George Chapman, who lived a century or so later - but a classical scholar and poet. Harkness cleverly weaves these and other well-known historical names into her story of witches, vampires, blood and lust.
Readers will find themselves transported to a time when witches were blamed for anything people couldn't explain. Harkness' descriptive prose will have you right there among the very rich as she takes readers to Queen Elizabeth's court.
Then you get a taste of the opposite side of life as she takes to the streets and back alleys.
I asked her some questions about her writing.
I HAVE TO ASK THIS ... DO YOU BELIEVE IN WITCHES AND VAMPIRES?No one has proven to me that they don't exist, so I'm reserving judgment.
WHEN YOU STARTED TO WRITE A DISCOVERY OF WITCHES DID YOU KNOW IT WASN'T GOING TO BE A STAND-ALONE NOVEL?Yes. The story of Diana was always a story in three parts. At first, I thought it would be one novel with three parts in it.
Then I got to page 400 of part one and realised it was probably going to have to be three books. Or it would have to have its own wheeled carrying case.
WHAT CAME FIRST FOR YOU, THE CHARACTERS OR THE PLOT?First there were pragmatic questions (Where do these creatures live? Work? How do they go out on dates?) and then there were characters. The plot unfolded more gradually.
WHAT'S THE SECRET TO A BELIEVABLE CHARACTER?I'm not sure. You should probably ask the readers. I think the most important thing is that the author has to feel they are real.
If the author is holding them at arm's length, or not thoroughly convinced, I think the reader will know.
HOW DO YOU CONVINCE READERS THEY ARE IN THE 1500s?What I tried to do was to use historical detail like seasoning in a soup. I found that a little historical detail went a long way and tried to focus on small details, rather than giving complete rundowns of major historical events.
WHAT RESEARCH DID YOU DO?Because I'm a historian, a lot of the research was already done. I've been studying the 16th century since 1982, and I have lots of notecards. It was still humbling to discover what I don't know, though.
I'm a historian of science, so I had to do a fair bit of reading on early modern ships, warfare and some aspects of daily life (like what foods one ate in Northern Europe in November).
ARE FANS GETTING MORE OF DIANA AND MATTHEW?There will be a third volume in the trilogy.
HOW DO YOU CELEBRATE FINISHING A BOOK?
I catch up with all my friends and family. They tend to get ignored when I'm in the final weeks of frenzied writing.
WHAT IS THE FIRST BOOK YOU CAN REMEMBER READING?Anne of Avonlea. I'm sure I read books before that, but this was a book that my mother shared with me and I adored it.
FIVE TOP TIPS FOR WANNABE WRITERS?Write every day. Go out and see and do things - writers have to have experiences to write about after all. Ask questions. Listen to the answers. And the most important tip: don't lose your sense of wonder.