Every non-fiction library book has subject headings assigned to classify it. Sometimes using key words will find you books on your subject, but except for very specific searches with distinctive words, using subject headings (known as LCSH's, or Library of Congress subject headings) will usually retrieve more books than just a keyword search.
Use key words to get LCSH's for better results. See the PowerPoint on this search strategy.
Also, remember to take your searches, if you like, from the local to the national/international level, by using Worldcat. If we have four or five books on subject x, and you want or need more information, search Worldcat using the same LCSH's you used for our catalog, and if you find books we don't own, you can order them by using Interlibrary loan (ILL). PowerPoint presentations on using Worldcat and ILL are on the home page of this guide.
The "standard" Library of Congress (LC) subject heading for the Spanish Civil War is Spain--History--Civil War, 1936-1939. The UMass catalog has more than 2,000 books on the subject; the combined Five College 'book count" is almost 3,000.
In alphabetical order under the "core" subject heading are sub-headings on every aspect of the struggle, such as "Aerial operations," "Art and the war," "Fiction," "Foreign public opinion," "Literature and the war," and a significant sub-heading, "Participation, American, British, French, German, Mexican, Russian," etc.
A search in WorldCat under the subject heading turns up more than 18,000 "hits"--books, films, journals, archivel sources. music, etc.