The Populist-Progressive Eras
Carl H. Chrislock, The Progressive Era in Minnesota, 1899-1918 (1971) -- a detailed study of the successes (and shortcomings) of the progressive movement in Minnesota.
Carl H. Chrislock, Watchdog of Loyalty: The Minnesota Commission of Public Safety During World War I (1991) -- this study focuses on how conservative forces in Minnesota politics used the First World War to suppress progressive, ethnic, and labor movements in the state.
Agnes M. Larson, John A. Johnson: An Uncommon American (1969) -- nice biography of Johnson, one of the first Scandinavians to win high office in Minnesota, who as governor pressed for progressive reforms.
Bruce L. Larson, Lindbergh of Minnesota: A Political Biography (1973) -- very good biography of Charles Lindbergh, Sr., the father of the famous flyer and a major figure in Minnesota's progressive movement.