When I worked in a bookstore, I constantly monitored the bestsellers to get an idea of what was popular among readers. I wanted to know, for my own writing, what readers found interesting and why. It’s the dream of most writers to one day see their name on that list, though for my own part, I don’t see too many historical fiction, medieval, fantasy type novels making that list, and that’s okay. I would rather write something I’m interested in and something I enjoy than try to find those “hot” topics and write with the intention to purely make money. You’re chances of making a living as writer are not that good anyway, so you might as well write about something you’re interesting in. For me, that’s medieval history.
Your particular genre might not be featured on any best seller lists, but it does have an extremely loyal following among those who do keep up with the genre.