Use the Advanced Search Subject option and limit by Material Type.
Other useful searches:
You can also add the name of the play e.g.
In the Barr Smith Library we classify works by Shakespeare at:
The number given to works about Shakespeare is:
A letter will follow the Z, which is the initial of the author's last name, and other numbers and letters. For example, Stephen Orgel's book
The Authentic Shakespeare: and Other Problems with the Early Modern Stage (New York: Routledge, 2002) has the call number