I don't know much about the TV Series but I'll try to explain my best. The TV Series includes many characters created by itself. For example Jackie Torrance (she is not present in King's any stories) and many others too. I don't much about Season 1 but Season 2 focuses on King's novel Misery.

King's novels that are set in Castle Rock include Misery, Bag of Bones, Cujo, Needful Things. Maybe you try to read them to understand more about the town.