The Farmer-Labor Party and its History
John Earl Haynes, Dubious Alliance: The Making of Minnesota's DFL Party (1984) -- a careful study of the links between the Farmer-Labor party and the various labor groups in Minnesota, and the influence of socialist and communist thought on the Farmer-Labor movement prior to its merger with the Minnesota Democratic party.
Robert L. Morlan, Political Prairie Fire: The Nonpartisan League, 1915-1922 (1955) -- a fairly comprehensive history of the nonpartisan movement which provided a strong impetus to the formation of the Farmer-Labor party in Minnesota.