In Doherty’s 14th mystery featuring Brother Athelstan, The Book of Fires, the medieval English priest must solve another impossible crime.

As the author of several historical mystery series, how do you decide which periods to write about?

The periods I have chosen are those I feel most comfortable with, those of which I have a sound knowledge and a good grasp of a particular society and culture. Human nature doesn’t change, and murder, like charity, is a common currency in all societies and places. My aim is to create a true murder mystery, but I also see my stories as historical novels. My knowledge of medieval history comes directly from my formal education, as does my love of Rome and ancient Egypt.

How did the Athelstan books come to be?

The idea of a medieval parish in the city slums appealed to me. We have the feast days, the rituals, the baptisms and the burials, the eccentric characters, the strange visitors, with more than a shadow of London’s underworld cast over all of it. Athelstan is a dedicated Dominican friar, who, because of his deep perception of the human heart and his sharp observations of all that is happening around him, is also clerk to the investigative coroner.

What was the political backdrop of the time like?

The glorious years of the Black Prince are over, and England has withdrawn from France. French corsairs attack the south coast. The Papacy itself is divided and the Catholic Church is beginning to confront serious challenges to its authority. In England, the young king is merely a boy overshadowed by his sinister, enigmatic uncle, John of Gaunt. London is still a very wealthy city with its powerful burgesses and richly brocaded mansions. However, there’s a storm coming. The dispossessed are beginning to protest against poor government, high taxes, and the excesses of the rich.

How do you avoid repeating yourself after so many impossible crimes?

I admit I have to work harder, be more cunning and innovative. I do spend hours pondering how to murder someone. I admit that sounds terrible coming from a Catholic head teacher, yet it’s the most relaxing exercise and so easy to focus on. I puzzle constantly. One morning on the tube, a young lady asked me—because she could see me deep in thought—what I was thinking about. Without reflecting, I replied, “I am plotting a murder.” Naturally, she got up immediately and walked away. I suppose my mind can swiftly turn that way. Recently, I was sitting in a hotel foyer watching people taking and depositing room keys, and I thought, How could that be used? I hope it’s a habit that never dies.