According to Wikipedia: "a secondary source is a document or recording that relates or discusses information originally presented elswhere." Generally, a secondary source pulls together all of the primary material, and synthesizes and evaluates that information in a new form. Some examples:
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HathiTrust is a smaller - and possibly higher quality - version of Google Books. Books published before 1923 can be viewed online in their entirety. Unlike Google Books, HathiTrust provides online access to both historic and current U.S. Government Documents.
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