Specific to Broad
Say you are interested in reading accounts from men who served on U-boats. You would want to search for books with primary source materials about:
World War I
World War II
Try a variety of terms. Look to see how the catalog or database lists the subject.
Related, Relevant Terms
Don't expect that one search will bring up everything that is relevant. Suppose you were interested in the history of birth control. All of these subjects might be of interest:
human reproductive technology
Do you know names of individuals, organizations, or other bodies involved in the issue? Use the names as search terms.
There's a really nice description of primary sources and ways to find them in The Information-Literate Historian
Thomas Cooper Library, Reference Dept. D16.2 P71 2006 (pages 92-135)
Libraries and museums may have digitized primary source materials. Great ways to identify virtual collections include: