Secondary sources are the published work of scholars specializing in the topic. Secondary sources include scholarly books, articles, and essays (both analyses by contemporary scholars as well as older analyses), surveys, criticism, comparative studies, reference sources, and works on theory and methodology; this is also termed the secondary literature. Eventually you will need to decide which interpretation makes the most sense to you and seems consistent with your primary sources, or if you wish to offer a new interpretation.
When we talk about secondary sources, most of the time we are referring to published scholarship on a subject, rather than supplemental material (bibliographies, encyclopedias, handbooks, etc.). Secondary literature is published in both book form and as articles in periodicals, either in print or digital format. (Digital format includes both reproduction of print material online and original e-text.) This scholarship is analytical and interpretive. It may synthesize the work of other historians to present a totally new interpretation. More likely, it offers a new reading of previously analyzed sources or presents an analysis of previously unknown sources.
Hence, you use secondary sources to identify the main currents of thought on your topic. Which historians have taken up this topic and what were their main arguments? How has our understanding of the subject changed with shifts in the predominant methodologies and theoretical perspectives in the historical profession?
To identify secondary literature, you can search the online library catalog to find books or search article databases and online bibliographies to find journal articles, book chapters, dissertations and more. Some useful article databases for history are:
You can also consult book-length bibliographies, either general overviews such as the American Historical Association Guide to Historical Literature, or specialized bibliographies such as The Harvard Guide to African-American History.
You can use the article databases listed above to look for review essays (also called review articles) in which a historian who specializes in the subject analyzes recent scholarship; you may find helpful overviews that appear as chapters in collections, journal articles, or even monographs by using the keyword historiography or historiographic; you can read about the topic in a subject encyclopedia and look at the bibliography at the end of the entry; and you can find a major work of scholarship on the topic and follow up on the sources used by the author (footnote tracking).
Most of the time you will find the secondary literature you need by using the online catalog, the appropriate article databases, subject encyclopedias or bibliographies, and by consulting your instructor.
Most academic research today is at least somewhat interdisciplinary. For historians, theories and methods are sometimes borrowed from other disciplines such as philosophy, literary criticism, and anthropology.
Furthermore, the publications compiled in the service of other disciplines very often serve as valuable source material for historians.
Check these links for helpful introductions to research resources for related disciplines: