It's been over a decade since I was required to read Green Centuries, so I am not prepared to offer much other than high praise.
Dr. John Willson of Hillsdale College preferred to assign novels instead of textbooks for his course on the History of the American West, and Green Centuries was, I think, the first of them. It's a long book, so like most college students I was dismayed with having only a week to read it. However, I finished it in only two days because I loved it.
If you like this genre, then you must read this book. I also recommend A.B. Guthrie's "Big Sky," and Vardis Fischer's "Mountain Man."