
On an unrelated note, Gwynne's book is fine as a fantasy story, but it's very badly regarded from the perspective of narrative history. Hämäläinen's Comanche Empire is a much better book arguing a largely similar position. Don't take that as applying to later books by the same author, sadly.
[1] https://yalebooks.yale.edu/book/9780300151176/comanche-empir...
It took a long time to get through (very dense, academic) but was a sprawling look at a powerful indigenous culture that I knew nothing about.
Plus, it was enlightening and a bit foreboding to learn how an empire could be at the very height of its power and then, through circumstance, climate, demographics, and imperial expansion be exhausted and destroyed in essentially a decade.
Amazon: http://www.amazon.com/Comanche-Empire-Series-Western-History...
The Comanche Empire is more of a history textbook.
IIRC, I found The Comanche Empire by looking at the bibliography of Empire of the Summer Moon.