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 for the holdings of many libraries, helps to check to see 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.
In addition to the content opened up by publishers and vendors on a temporary basis (see link to the RIGHT), there are many ebook titles that may be available to us temporarily or at a reduced price. For example, Ebook Central from ProQuest has opened up add'l. titles PU affiliates can access temporarily and individual titles are likely NOT going to be listed in the library catalog. Similarly, EBSCO has made add'l. content available to varying degrees from specific publishers. See this list for more information. Also, Taylor & Francis/Routledge has many more titles available than what we had access to previously (that link is to the Museums & Cultural Heritage titles, but there are other subjs. available), and some of them are available to access/"checkout" via VitalSource (set up a free account).
There is a second HathiTrust option now available to Princeton users on a temporary basis. Several million titles have opened up via the HathiTrust Emergency Service. One should find links to these titles in the library catalog, but can also search the native HathiTrust database itself (select LOG IN at upper right and login as a PU patron). If using the latter, uncheck the "full-view only" box, and select CATALOG instead of FULL-TEXT. In the library catalog, one will see "temporary digital access from HathiTrust," and will have the ability to view a title in full for one hour at a time. These titles are all owned by Princeton in print, but don't include titles we had access to from Columbia and New York Public. NOTE: these titles will NOT be filtered in an ONLINE only limiter using the library catalog. For more information, see the story on the PUL website.