Black Elk Speaks is considered an almost "sacred text" along some because it's told by someone who lived in a time when the plains Indians were still "free".
Here's the blurb on Amazon:
Black Elk Speaks is widely hailed as a religious classic, one of the best spiritual books of the modern era and the bestselling book of all time by an American Indian. This inspirational and unfailingly powerful story reveals the life and visions of the Lakota healer Nicholas Black Elk (1863-1950) and the tragic history of his Sioux people during the epic closing decades of the Old West.
In 1930, the aging Black Elk met a kindred spirit, the famed poet, writer, and critic John G. Neihardt (1881-1973) on the Pine Ridge Reservation in South Dakota. The Lakota elder chose Neihardt to share his visions and life with the world. Neihardt understood and today Black Elk is known to all.
Black Elk's remarkable great vision came to him during a time of decimation and loss, when outsiders were stealing the Lakotas' land, slaughtering buffalo, and threatening their age-old way of life. As Black Elk remembers all too well, the Lakotas, led by such legendary men as Crazy Horse and Sitting Bull, fought unceasingly for their freedom, winning a world-renowned victory at the Little Bighorn and suffering unspeakable losses at Wounded Knee.
I'm afraid I didn't quite get that feeling from it when I read it (and, since it had such a glowing reputation, ended up being disappointed with the book). It might be one of those books were people bring a lot of their own preconceived opinions to the story, and take away what they want to take away. I read it years ago; I'm sure if I read it again I would have yet another opinion.