This guide covers general reference sources at Princeton University Library with a focus on general online reference resources and general print reference resources located in the Trustee Reading Room of Firestone Library. It also has information about types of reference works and how you might use each in your research.
In addition to the general reference sources covered here there are many subject-specific reference resources. The best way to learn about what is available in your subject area is to visit the research guide for your discipline or get in touch with your subject librarian.
These are large, multidisciplinary online reference resources and can be a good starting point for getting background information if you are new to a topic. Each database contains hundreds of subject-specific works and will search across those works when you enter your search terms.