Since I haven't read it, I can't be sure. But it might have characters like the workers in the salt house, or a family member who has a special connection to the salt house. There could also be a local trader who deals with the salt produced in the house.
Not having read 'Salt House A Novel', it's hard to say exactly who the main characters are. However, one could assume that there might be a character who is in charge of the operations at the salt house. There could also be a young person who is learning the ropes of the salt - making business in the salt house and their journey could be a major part of the story. And perhaps there is an outsider who comes to the salt house and changes the dynamics there.
I don't know as I haven't read the book. Maybe there could be a salt house owner as a main character.