I must admit I’m not a big fan of historical mysteries as a rule. I layed out the main reasons why in this old post so I won’t repeat them here. I’ll add another one, however: most historical mysteries are not historical enough.
There is no escaping, whatever the genre’s « origin story » one adheres to, that the genre is a product of the Enlightenment. There were stories about crime before that, and you can even find some detection in the Bible, but it took centuries, even millenia, for the genre to coalesce into its modern form. That evolution closely followed that of civilization as it progressively embraced reason and science as its guides (I’m aware it’s more complicated than that, but a more accurate summary would triple the length of this post)
This is where the problem, or at least my problem, with most historical crime fiction comes in. How do you write a mystery set in times when neither the genre or the conditions for its appearance existed? Few writers and fewer readers seriously ponder this question or even are aware of it and yet it is a crucial one.
I’ll spare you the oft-used L.P. Hartley quote about the past as a different country but it points out to an incontrovertible truth: people from even ten years ago don’t quite see things as we do; in some cases they don’t even quite think the way we do. Our logic and that of, say, a Middle Kingdom Egyptian have nothing in common – the premises, line of thinking and resulting conclusions are irremediably different. Had crime fiction been around back in King Tut’s time, chances are that its version of Sherlock Holmes would have used methods alien to us. Also, the order to be restored in the end would not necessarily be ours, as « justice » too is an evolving concept.
Another, if less important, area left unexplored and unquestioned by historical crime fiction is what I’ll call the formal question. What would a mystery or a crime novel actually written at the time look like? Literary conventions have changed a lot over the centuries and probably will change again a lot in the future as they are inextricably linked to mentalities and mental representations. As a result, it is very unlikely that a detective story written in the Middle Ages would read like an Ellis Peters novel. It would be nice to have books that try to be as close as possible not only to the period detail and mores but to the actual literature of the time. That’s what Robert Van Gulik did with his Judge Dee stories.
As said above, few writers care about such complexities. Most of them either stick to « easy » periods like the Victorian era – though there would be a lot to be said here too – or simply dress up a modern plot and modern characters with period clothes and that’s how we get Roman or Renaissance sleuths thinking and behaving centuries ahead from their time. This often makes for enjoyable reading but this is not what I call historical fiction but rather fiction with a historical setting, which is not the same thing despite an almost similar wording.