The National Association of Secretaries of States provides links to each U.S. state's agency regulating voting and elections, usually with access to historical voting results and registration data. (You also can scan down the NASS homepage to "Find Your Secretary of State.")
A similar list is available from the U.S. Federal Election Commission.
The Census Bureau regularly conducts surveys on "voting and registration by various demographic and socioeconomic characteristics" and provides both recent and time-series data.
Detailed examination of the history of U.S. elections and the electorate through the 20th century.
Atlas of the 2016 Elections by Fred M. Shelley (Editor) et al.... In addition to the presidential election, the Atlas has full coverage of other important races, including United States Senate and House of Representatives, state races, and local and state referendum. Illustrated with over 100 meticulously drawn full-color maps, the Atlas will be an essential reference and a fascinating resource for pundits, voters, campaign staffs, and political junkies alike.