The Earth Is Weeping
This is surely the definitive work on the Indian Wars in the American West (i.e. west of the Mississippi) from the end of the Civil War to the final defeat of Geronimo in the 1890s—the last 40 years of a 400-year confrontation. It is essentially a military history, detailing the wars skirmish by skirmish. There was only one real battle, the infamous Little Big Horn (Custer’s Last Stand) of 1876. Otherwise, it was the same thing over and over: broken treaty/warpath/pacification/new treaty.
Cozzens gives an interesting account of the Indian method of warfare and an even better account of the Union army, including its oft-neglected Indian auxiliaries. Despite the title, the Indians are not romanticised, nor are they all treated as hostile savages. There was more peace than war. There are very good maps. An invaluable source of reference.