A secondary source is a work that interprets or analyzes an historical event or phenomenon. It is generally at least one step removed from the event. A recent article that evaluates and analyzes the relationship between the feminist movement and the labor movement in turn-of-the-century England is an example of a secondary source; if you were to look at the bibliography of this article you would see that the author's research was based on both primary sources such as labor union documents, speeches and personal letters as well as other secondary sources. Textbooks and encyclopedias are also examples of secondary sources.

To find secondary sources, look in the library catalogs (for books and other monographs) or periodical indexes such as

Historical Abstracts or America: History & Life