I think my personal preference would be towards Monsterberg II, Setting Books, and then Magic expansion.
Based on how you say sales are going, though, it sounds like Setting Books might be the better balance between the history and the gaming crowd. After all the Osprey books weren’t gaming books (and had plenty of issues on their own, in some ways understandably since they sometimes took a rather broad time and place to try and make a succinct history book on, so it’s probably to be expected that you’ll lose a lot of nuance that way) but they were used by a lot of gamers I know to fill in flavor for games. So in a way a more focused setting book can do double duty as not just a game reference but an historical one as well… sort of like some of the GURPS setting books were for some folks I know (who also loved to use them for other game systems even if they didn’t like playing GURPS!) So in that sense you might be able to create books that appeal to both crowds.