Secondary Sources
Oxford Bibliographies Online are literature guides in several humanities and social sciences areas. They offer annotated lists of secondary source articles on particular subjects. Oxford Bibliographies by subject. Example: Pan-Africanism.
America: History and Life (1955- ) includes books, book chapters, journal articles, and book reviews on North American history.
Web of Science Citation Indexes allow citation searching, that is, starting with an article of interest and finding more recent articles that have cited it.
JSTOR allows simultaneous or individual searching, full-text searching optional. Advanced search is most effective.