I would say you need two sets of lore, one for each perspective(Cowboy/Indians).
It has to be more in-depth than just simply collecting books and journals, while that plays a part in it there needs to be some sort of lore progression system in place and possibly a unique and interactive story line for both the Cowboys and Indians.
E.g, there could possibly be an NPC for the Indians who sits around a camp site and upon interacting with him/her the player gets told a piece of the lore and this would sort of be the start for the story line, in order to track their progress players can access a lore menu and open any journals/book they have received to re-read or just view how many pieces of lore they are missing.
An idea I always thought would be neat for something like this was that there would be certain places/characters that they would unlock while learning about lore which would sort of go in line with having a whole interactive story line.
There is a lot of things you can do with this, do you use Discord or anything? Would love to discuss some ideas with you.