Using this resource, you can look up authors' brief biographies, works and dates of publication, as well as information about major scholarly issues involving interpretation of their work.  You also can look up individual works, characters (by name), and a host of other useful literary trivia that comes in very handy when trying to do scholarly analysis under pressure.  Usually, information in a "companion" volume is not considered "scholarly" (or quotable or citable), unless as an indicator of some "normal" opinion that you are taking issue with.

        For more specific guidance in essay-length chapters on specific topics about an author or era, do a Title search in OLLI under "Cambridge Companion" and you will see a large series of volumes dedicated to representing the standard scholarly opinion.  These are even better for citing as evidence of "normal" scholarly understandings about a work or author or era.  To give you an example of this series' level of coverage, the following are commonly useful to students of early literature:

[In the case of Dinshaw's "New Approaches" chapter, especially, keep in mind that "new" dates quickly--check the date of publication (2003) and be advised.  In all cases, prefer a more recently published "Companion"'s opinions to a more elderly volume unless as evidence of older opinion in contrast with newer views.]

See also:

And don't forget the value of using broader issues to inform your original interpretation of a primary source by getting some background on religion, history, genres, and other cultural context: