Drama for Students is a 30 volume encylopedia 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 playwrite 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 formated in much the same way as Drama for Students.
Here are some digital and print resources for locating major American 20th century plays.
Try searching the name of your play and author together as well as the playwright alone to see generally what kind of literature exists.