The Nebraska State Historical Society has the best collection of published biographies. There is no statewide biographical index or major manuscript collection. State, regional, and county histories often have biographical sections. Some representative biographical encyclopedias for Nebraska are:
Compendium of History, Reminiscence and Biography of Nebraska. Chicago, Illinois: Alden Publishing Company, 1912. (FHL film 1000179; similar volumes are on FHL film 1000180.) http://www.usgennet.org/usa/ne/topic/resources/OLLibrary/Comp_NE/index.htm#Table%20of%20Contents
Baldwin, Sara Mullin and Robert Morton Mullin. Nebraskana: Biographical Sketches of Nebraska Men and Women of Achievement. Hebron, Nebraska: Baldwin Company, 1932. (FHL book 978.2 D3ba; film 1036245 item 4.)
Sheldon, Addison Erwin. Nebraska: The Land and the People. Three Volumes. Chicago, Illinois: Lewis Publishing Company, 1931. (FHL book 978.2 H2s; film 1000180.) Volumes 2 and 3 are biographical.
Morton, Julius Sterling et al. Illustrated History of Nebraska. Three Volumes. Lincoln, Nebraska: Jacob North and Co., 1905-13. (FHL book 978.2 H2m; film 1036256.) The Nebraska Historical Society has an every-name card index to these volumes on microfilm.