Airth’s The Decent Inn of Death (Penguin, Jan.) draws retired Scotland Yarder John Madden into his former superior Angus Sinclair’s investigation of a suspicious death.

Where did the inspiration for the series come from?

It grew out of my interest in WWI, which was sparked by my discovery of a scrapbook my grandparents had kept about one of their two sons—my father’s elder brother—who died in that war. While reading about it, I got the idea of a Scotland Yard detective not wholly recovered from his time in the trenches who must track down a killer who’s equally damaged.

Where did you get the plot of The Decent Inn of Death?

I can’t remember exactly, but I wanted to set the book in about 1952, and I recalled how obsessed the British still were with WWII, and wondered if I could put an escaped Nazi war criminal at the center. I came up with the idea of suggesting the killer might be just such a man while leaving the question of his identity open until near the end.

Why have you spaced out the books, in terms of historical setting?

I didn’t want to write “another case for Inspector Madden” kind of book where the hero detective never seems to get any older. Both Madden and his friend and former superior, Angus Sinclair, age visibly as the series progresses, and we learn almost as much about their lives as we do about the crimes they are bent on solving. I knew I was setting myself a problem by fixing for Madden to quit the police force, but I’ve enjoyed the challenge of somehow involving him in these tales in what I hope seems a natural manner. I also enjoyed writing about the changing social scene in Britain as the years went by.

You’ve been quoted as saying, “Each of the Madden novels has a particular theme.” What’s the theme of Decent Inn?

Aging. Angus Sinclair, who shares equal billing with Madden in this tale, fears that age has finally caught up with him as he struggles to make sense of the puzzle he’s been presented with. But he soldiers on, and as the story nears its end has to face the prospect of his own death. This ties in with the title of the book, which comes from the G.K. Chesterton poem “Before the Romans Came to Rye,” which I first learned as a boy. Although he’s shaken to his depths by the thought that his time has come, the chief inspector rallies, and by the end he has come to accept whatever fate has in store for him.