Being an Australian, my knowledge of the American west and the Native American peoples is coloured by 'Cowboys and Indians' movies and TV series. Though I've read a little about the American west during the 1800s, it has been from a modern perspective.
The value of a book like this is that it is written by someone who was actually there. Sure its long winded, and gets much into the facts and figures, but then it is a diary of a soldier's life during a traumatic period in the U.S. with the Civil War in progress, and the problems associated with the relationships with several of the plains Indian tribes at that time, especially the Sioux and Cheyenne.
What's fascinating is the day by day activities of these hardened soldiers and their constant confrontation with exposure to the elements, the movement of civilians and Mormons into these regions, and of course how the Army and government were dealing with the Indian issues. The writer's attitudes are raw and seem honest about his feelings, perceptions about what is happening about him. He shows very mixed feelings about Indians, and settlers, Confederates allied to the Indian tribes, and it says something of how people of those times thought about what was happening in their world. Mention of contact with now famous people occasionally arise as does the perception of how close the Union was to losing the war based on public perception even among the ranks of Union soldiers who felt a negotiated peace should have occurred by 1863 with the Confederacy. And the activities and strategies of both the Army and Indian tribes is quite enlightening.
I found it a fascinated account, despite being long winded, and learned a great deal of how people of that time survived, thought, entertained themselves. The author describes a huge range of very interesting characters, people living in raw conditions, facing constant danger, yet eking out an existence. Its nothing like the 'Cowboys and Indians' film and TV portrayals. It is so much more and so very different. Takes an effort to stick with it, when the author rambles a little, but its a rich and worthwhile read for anyone wanting to really get a sense of what living in the 1860s US West was really like.