Usually researchers find more sources by looking at the footnotes in an article or book, but these will always be older than the publication you have in hand. Citation indexes like the Web of Science (which includes sections for the Social Sciences, Arts and Humanities, and Natural Sciences) are set up to search for sources cited in the footnotes of journal articles. This allows you to find newer articles which cite the books and articles you know are key for your topic. By relying on connections between authors and by moving forward in time, citation searching can open up new avenues of research.
Let's say you're interested in how African American families and communities have historically approached elder care.
Using the America: History & Life database, you try the following query:
There are 71 results, so you decide to narrow the historical period to 1850-1900.
Now you find a few interesting and relevant results:
Perhaps you're curious about workhouses and similar institutions, and wondering about conditions and care for the elderly residents.
Using the ProQuest Research Library database, you try the following search:
There are only 18 results, so you decide to take out the subject term and replace with two more parentheticals:
"elder care" OR health OR condition* (all text)
Now you have a larger set that you can narrow according to source type, document type, and so forth. You end up with a few highly relevant articles: