The Indian Ring by Don Bendell

THEY WANTED HIM DEAD...

A spy has infiltrated President Grant's administration. The best man to weed him out: Joshua Strongheart, a half-Lakota warrior/half-white lawman of national repute. But luck isn't on Strongheart's side. As soon as word gets out that he's on the job, a group of well-trained shooters are on his trail.

...BUT YOU CAN'T KILL A LEGEND.

Ronald M. Hartwell once worked for the Secretary of War, but he is now the leader of the Indian Ring—a deadly syndicate of criminals out to decimate the native world. For Strongheart, nothing could hit closer to home. But when Hartwell sends his most trusted henchmen to bushwack his opponent, there's only one thing left for Strongheart to do: Kill or be killed.