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Using reference books such as encyclopedias and historical dictionaries is an excellent way to begin your research. Encyclopedia and historical dictionary articles provide broad overviews of topics, critical questions and issues about topics, and bibliographies.
Search the USD Library Catalog for reference works. For example, enter United States History Encyclopedias as a subject search and browse the results.
Also, see the Research Gateway and filter by History. See the various e-reference works in the list.
The Hathi Trust Digital Library is a great tool for accessing digitized texts from other libraries.