The Library of Congress maintains a controlled vocabulary of subject headings. Subject headings organize books and other information resources into categories according to their major subjects. For example, here are the subject headings for Jefferson Cowie's Stayin' Alive: The 1970s and the Last Days of the Working Class.
These subject headings are subdivided, making them more specific with each subtopic. And because they're defined (i.e. controlled) by the Library of Congress, you can use them across different academic databases to locate sources. Try using a whole subject heading, or just part of one, to create narrow search queries. Cross-reference subject headings with topical keywords, as illustrated by the example below (WorldCat pictured).
Caveat searcher! When you construct a query like this one you may be inadvertently removing relevant results. It's often best to begin with broad keyword searches. Once you have a clear, well-defined sense of the resources you need, search by subject to home in on the most relevant stuff.
Remember to read the bibliographies of secondary sources! Doing so will help you discover the primary documents that support scholars' interpretations of historical events.