The library catalog is a good place to start to see what e-content is available to you in the form of books, e-journals (only some years may be available online), etc. (but NOT generally journal articles themselves). Do a Keyword, or Title Starts With search, and limit results to ONLINE under Access on the right-hand side. Browzine is a great resource for browsing e-journal content.
One may wish to install the browser extension Anywhere Access to enable access to PDFs available from the library wherever one is searching on the Web.
WorldCat, which is a catalog of the holdings of many libraries including and beyond Princeton, can help to determine if an electronic/digitized copy of a title exists (books). Results are organized by type. See tab at top for INTERNET if e-versions exist. The free version is available at worldcat.org .