(These techniques will work in both the library catalog and WorldCat.)
To be certain that you're looking at all of the books on a particular topic, it can be useful to search the official library subject for that topic. Sometimes the library subjects are not intuitive, sometimes they are. There may be more than one subject term that you'll need to look at for your topic -- both ethnicity and race, perhaps. You can use either the basic or the advanced search options to search for subjects, use the pull-down menu to select "subject."