PHI 501

Looking at the notes/bibliographies of articles you have already found is a tried and true way of expanding on your search for sources.

Copy/paste type the title or other info from the citation into the library search tool.  You can check for both articles and books this way.

While traditional bibliography tracing always takes you further back in time, Google Scholar lets you go forward in time by seeing who has cited an article after it was published. The Libraries have databases for citation indexing in the sciences, but I've found Google Scholar to be the best bet in any humanities field, as it pulls from both journal articles and books.