Secondary sources are research books and articles. For our purposes, it is convenient to divide these sources into criticism and reference materials. Criticism will make arguments about a text or a literary movement or a period of time, drawing evidence from many places. These kinds of sources will be your main partners in the scholarly conversion, worthy of citation in your paper.
Reference sources provide context and factual information to help orient yourself when developing your argument. Encyclopedias and timelines are good examples. While online reference sources such as Wikipedia can be helpful, the materials held by the library will be more reliable since they are vetted by scholars. Be careful what you consult.
The line can blur between critical and reference sources, but in general, you will not be quoting many reference sources in your paper unless the information they give is debatable. It can be confusing to determine whether a factual datum like a date is "general knowledge" which need not be cited or a contestable claim which should be quoted, so please contact your professor or a librarian if there is any question in your mind.
Below, critical resources are listed in the left-hand column, beginning with "Key Critical Sources." Reference works are included in the right-hand column. And of course search the catalog for reference and critical sources in print.
The resources below include some of the largest databases in English literature. They are often the first stop on a research project, but they should not be the last. They are neither comprehensive nor sufficient, and should complement normal catalog searches (as well as searches of more narrowly-focused databases).