Reference books are located on the first floor of the library, with the call numbers:
Cambridge Guide to Theatre
PN 1625 and PN 2035 are the two main places to look for theatre reference books. However, keep in mind that plays are literature, so some reference books will also show up in other areas. For example, most reference books about Shakespeare are found around PN 2892.
The Oxford Companion to Theatre
Dictionary of the theatre: terms, concepts, and analysis
Elizabethan Dramatists