When I decided to begin a new series about someone solving murders around the capitals of Europe in the 1930s, the first and the most important question became: who is that someone? Or, perhaps, sometwo?

I came across a fascinating book, Last Call at the Hotel Imperial by Deborah Cohen, which tells the stories of the young American men and women who flocked to Europe to see the world and become foreign correspondents. Many of them had no money, not much experience and no promise of publication of any of the articles they wrote. They were winging it – a key skill for journalists. Historically, many of the travellers to Europe from the United States had come from wealthy northeastern backgrounds and Ivy league colleges, but most of this crowd were from the midwest, from modest families and quite a few had attended the University of Chicago. They were writing about Europe for readers like them; editors soon learned that their articles sold newspapers.

Dorothy Thompson was one such hopeful journalist. Having bought a one-way ticket to London in 1920 with very little money in her purse, she somehow managed to interview a Sinn Fein leader who died in prison a couple of months later. She went on to become one of the most famous American foreign correspondents between the wars. There were a lot like her who travelled to Europe, including my very own Esme Carmichael.

She comes from Kalamazoo. I just love the word Kalamazoo. I've never really believed Kalamazoo was a real place. But I have visited it now, and I can confirm it exists. It's in Michigan and, disappointingly, it's not as boring as I had hoped it would be.

So I had my detective, my 'someone'. But did I need 'sometwo'?

I did; I do. My Icelandic detective series is about Magnus, my American-Icelandic detective, but it's also about Vigdís, his sidekick. I had always enjoyed writing scenes with the both of them.

So, who could be my 'sometwo?' He had to be different to Esme, a contrast. A man, then. Not American, but British. Older.

I grew up in a semi-feudal village in Nidderdale in Yorkshire, and while the dales have appeared tangentially in some of my books, this was my chance to write about someone who lived there. It would be handy if he had a bit of money, enough to stay in nice hotels and spend his way out of plot holes where necessary. A landowner then, a minor aristocrat with a small estate. Archie was a good name – Sir Archie to emphasise his social status. But why would he travel around various cities in Europe? And how could he team up with Esme?

A historian! That would work. I decided he would be a military man, a writer of biographies of various European generals. A man like that would need research help as he travelled. A woman like Esme would need money badly to tide her over until she got her articles published. Archie could employ her to help him and then they could come across a murder in Berlin or Vienna or wherever. Esme would be determined to investigate to write up an article.

I imagined Esme as young, energetic, resourceful but broke. Archie has 'issues'. And the big issue everywhere in Europe in 1930 was what today we would call Post Traumatic Stress Disorder but was then known as 'shell shock'. Millions of young me had died in the trenches in the First World War, millions more had watched them die and never forgotten what they saw.

Shell shock was everywhere, but always beneath the surface. If you look for it you can see it in the novels written during the period: in Sayers, in Christie, in Henry Wade, even in Virginia Wolf's Mrs Dalloway. It was in the minds of the politicians of all the countries of Europe who had lost sons and brothers in the war and who were desperate to avoid another one. It was behind the frozen expressions of commuting 30 and 40 year-olds on the suburban trains of London and Paris and Berlin. And my man Archie has a bad case of it.

He also collects incunabula, fifteenth-century printed books. It gives him an excuse to rummage around the old bookshops of Europe. He's trying to collect a book from each of the centres in Europe that printed them in the fifteenth century. I knew nothing about incunabula when a nice chap at the book dealer Bernard Quaritch suggested this plan. I know more about it now.

So I have Esme. I have Archie. It's 1930 and they're off to Berlin!