The Last Days of the Sioux Nation (Second Edition) by Robert M. Utley is a classic, but not necessarily the one book you’ll want to read about Native Americans of the northern Plains. But I put it here because it is currently dirt cheap on Kindle, and if your library does not have this 1963 title you may want to grab it up. I would recommend as a first read in this area of culture and history the recently published tour-de-force Wounded Knee by the amazing historian Heather Cox Richardson. Not on sale at this time but worth it.
I should mention that right now you can get some great deals on the Paperwhite Kindle.