Most library catalog records include what is called "subject(s)," or "subject heading(s)." (Excluding some works of fiction). On our Classic Catalog and WorldCat, subject headings are clickable links. Click on the heading relevant to your research topic to find more books written on the same subject.
Some index databases (such as the MLA International Bibliography and Academic Search Complete hosted by EBSCOhost) include a function to suggest or extract related subject words/topics from your search results. Use those to expand or narrow down your search.
An EBSCOhost example:
An increasing number of catalogs and databases implement boxes and links to narrow down your search results IN THE LEFT-HAND SIDE COLUMN of the results screen (called facets). Keep an eye on the left-hand side column and use those to make your search results less threatening.