Jo Walton, best known for her fantasy fiction, concludes her alternate history trilogy with Half a Crown, which imagines a world in which England and Nazi Germany made peace.

How different was writing these books from your fantasy work?

For me, it was exactly the same. I have the world and the characters and what I’m doing. There are always things to check, whether it’s which London station you use to go somewhere or how far a trireme sails in a day. My fantasy books have politics in them, too, it’s just that nobody takes any notice in fantasy. Also, all my fantasy books are closely based on history. I like doing that. It gives you somewhere to start.

How did the series come about?

The idea for the alternate world came from rereading Josephine Tey’s unsettling Brat Farrar, which appeared to be set in the early 1950s, but Hitler was at the Channel and the novel’s privileged people were going about their lives as if it didn’t really matter. I wanted to unsettle my audience in a similar fashion. I’d also reread George Orwell’s Collected Essays, Letters and Journalism, and a lot of the world-building came from what Orwell was afraid was going to happen. Finally, at the time I was writing, I could hardly look at the newspaper or a blog without seeing things such as the revelations about Abu Ghraib. I was certainly more alert to the question of how good people come to go along with evil things because of current events.

Half a Crown and its predecessors, Farthing and Ha’penny, are also unconventional murder mysteries.

I wanted to write mysteries where everything wasn’t put back in the box at the end. I’ve always wanted to subvert the way mysteries have these absurd murders as interruptions, and the right person is always brought to justice and order is restored.

What are the advantages of alternate history fiction in getting readers thinking about political issues?

In an alternate history, people think they know what they’re getting, so they’re more relaxed, and it’s possible to get them around a blind corner. You have all these country houses and cups of tea and flowers, and that sets up expectations, so that the realization that “this isn’t actually nice” can be a surprise.