In The Mirror Man (Knopf, Jan.), the eighth Joona Linna mystery, Kepler, the pen name of Alexandra and Alexander Ahndoril, pits the Finnish detective against a sadistic killer.

How has the writing process evolved since The Hypnotist, the first book in the series?

Alexandra: We have learned to cooperate better and better. We don’t do any detours now. We understand much better working out the plot before writing.

Alexander: The writing process is more effective. We’re trying to always get better, so it takes longer, even though we get more effective as writers. The Hypnotist was just our passion about writing together. Then, since this became a series, we tried to find the benefits of not leaving your characters when a book is finished. The hard part is to continue the story without spoiling earlier books.

Has Joona changed since The Hypnotist?

Alexandra: The interesting thing is that we thought that we would get to know our main characters better and better for each book. But still, there are sides of them that we don’t know anything about, and Joona, for us, he’s still kind of a mystery.

Alexander: I think that’s the way with friends or people you love—that the more you know about them, the more you want to know, so you just get more and more questions. Joona has surprised me with a moral discussion that he has all the time in his mind: is he really making the world a better place, or the opposite?

Alexandra: I think that the moral dilemma for Joona has become more and more acute in each book.

How do you explain the appeal of your books, given their darkness?

Alexander: For us, writing these books is a way to take care of our own fears. We put our fears into the story, trying to make them better because they start with a lot of evil, horrifying things, but actually journey from chaos to order in each book. It’s some kind of justice, some kind of answers, and that’s a satisfying journey for us as writers, but I think, also for readers, to have this. In real life, you read newspapers and it’s so cruel, the world, and you never ever get any answers.

Alexandra: We pick a problem—a thing that really scares us, like sexual violence in The Mirror Man—and isolate it, make it smaller. We do cry a lot and sometimes have to walk away from the computers because what we’ve written is so intense. Also, we give the readers the same clues as Joona has, so it’s possible to solve the case in the way he does.