There is a lot of difference between a historian and a historical writer. If you want facts, the best sources are the reports of US Army officers of the time, who much of the time acted as mediators between tribes, while often disregarding “kill them all” orders from Washington.
But the fantasy approach leads nowhere.
Allan Eckert doesn’t write fantasy which, ironically, is exactly what Twain was writing. Also try some Royal B. Hassrick, Mari Sandoz and others who actually knew Indians who lived on their own. Oh, and Lewis and Clark’s accounts.