It could be either antique or imported merchandise and other rare items. I also thought about having the family being in the service of an aristocratic family who is part of a secret society, kind of like the free masons or an occult lodge, etc. Something that has to be kept secret since christianity is obviously the social norm in this time period. The bookstore could also serve as a front for any shady dealings or messages sent between the family and agents of the lodge the lodge etc.
That’s the sort of stuff I’m interested in playing. 😈