If you have an ereader or don't mind reading online Project Gutenberg is a free choice.
Mammals of Northwestern South Dakota by Kenneth W. Andersen
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http://www.gutenberg.org/ebooks/33659
Books on Farm life -- South Dakota -- History
Land of the Burnt Thigh by Edith Eudora Kohl
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http://www.gutenberg.org/ebooks/24352
Sioux Indian Courts by Doane Robinson
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http://www.gutenberg.org/ebooks/26021
The Project Gutenberg EBook of American Missionary, Vol. 45, No. 2,
February, 1891, by Various
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http://www.gutenberg.org/files/16060/16060-h/16060-h.htm
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