Empire of the Summer Moon (S.C. Gwynne)
It's the story of the rise + fall of the Comanches in the late 1800s. So far, it's an unbelievable story, and if you're generally interested in the post Civil War era as the U.S. expanded into the west, you'll enjoy it.
Lonesome Dove (Larry McMurthy) is possibly my favorite fiction book of all time, and that got me increasingly interested in this time period.
I also started reading some of Cormac McCarthy's work after Lonesome Dove. Blood Meridian also takes place during this general time period, and it's possibly the weirdest book I've ever read.