Drama for Students is a 30 volume encyclopedia Van Wylen has as a part of its eReference collection. It contains essays on over 550 plays and provides summaries and analysis of themes and historical context, as well as bibliographies of additional resources. The easiest way to see if it contains the play you are researching is to do an Advanced Search in Gale Virtual Reference. Enter the play or playwright in the first search box and "Drama for Students" in the second, and change the dropdown to "Publication Title."
For those of you considering a very recent work, Gale also has a volume called Twenty-first Century Drama, that is formatted in much the same way as Drama for Students.
This titles are located in the print reference section, on the 1st floor of Van Wylen. These titles are good for getting oriented to a writer or theatre in a specific country, or for selecting a play to work with.