Doesn't have to be one of the big ones: historical era or culture, it could be a smaller part of what will be your WIP.
I'm currently reading Slavery in the Roman World (Sandra R Joshell), because my Roman spy has gone undercover as an escaped slave, and things are about to go horribly wrong . . .
I'm currently reading Slavery in the Roman World (Sandra R Joshell), because my Roman spy has gone undercover as an escaped slave, and things are about to go horribly wrong . . .