Tertiary sources, which you can think of as broad background sources like textbooks or encyclopedias, have a tendency to be overlooked - but they can be incredibly valuable. Some of their uses include:
You may also find overviews of topics in books. Search in the library catalog as well as WorldCat to get the widest array of possibilities.
Books held in Swem or available online.
Sometimes the Libraries don't have books on your topic - but that doesn't mean they don't exist. Use WorldCat to search for books held worldwide, which you can request through Interlibrary Loan.