In The Mitford Murders (Minotaur, Jan.), Downton Abbey expert Fellowes, the niece of the show’s creator, Julian Fellowes, kicks off a series featuring the members of the aristocratic Mitford family and providing plausible solutions to real-life crimes.

What inspired you to write The Mitford Murders?

I wanted to try fiction, and I knew that I would write something set in the 1920s; it made sense of the research I had done for the Downton companion books, which were about the making of the series but also to explain the context in which they were set, the social history and the family stories that inspired Julian. So when Ed Wood of Little, Brown approached me and asked if I’d like to write a crime novel set in the 1920s featuring the Mitford sisters, I leapt at it.

When did you first learn of the murder of Florence Nightingale Shore?

Ed suggested it, so I looked into it.

I then found a possible connection between the murder and the Mitfords, which set me off on the plot. I also became fond of Florence Nightingale Shore, the more I got to know of her and her brave work, and liked the idea that this book could bring more attention to her and her fellow war nurses.

Had you known about the Mitford sisters before you started working on the series?

Yes. When I was 21 years old, I first read Nancy Mitford—Love in a Cold Climate and The Pursuit of Love—and adored her writing. Over the years, I’ve read more of her books and dipped in and out of the copious books that compiled the letters the sisters wrote to each other. They are a compelling unit, and have been an endless source of fascination for me and many others. I was even lucky enough, when I was deputy editor of Country Life magazine, to stay with Diana Mitford’s son, Desmond Guinness, in his castle in Dublin.

Was there something about the period you hadn’t expected?

I think it’s the ongoing realization that we are no different to the people of the past—we only wear different clothes and live in a different context. Essentially, if you put anyone living now in a time machine and sent them back 100 years ago, as people they would not change hugely. This also helps to explain something of the cyclical nature of our world. I think there are many parallels between today and a century ago—the dominance of new and constantly developing technology, the rise in polarizing politics, the feminist wave.