For collections of plays, try:
For collections of children's plays specifically, use instead:
For books about playwrights or plays (criticism or about plays as a literary form), some subjects to use include:
For collections of monologues, some subject headings to search include:
For books about monologues (criticism or other works about monologues as a literary form), try:
For browsing, here are some call number ranges that contain different types of plays and monologues: