For individual playwrights, search by their names in inverted order in the subject index. Look for entries in the subject index that have the subdivision --Criticism and interpretation.
Encyclopedias and dictionaries remain valuable sources of bio-bibliographical information and concise critical commentary on authors and types of literature. Consider keyword searches such as:
Bibliographies can provide useful citations to articles, books and other resources on a topic. Older resources cited in printed bibliographies may not be indexed in more modern electronic resources.
See for example these subject headings:
A bibliography focusing upon Australian Aboriginal, Canadian First Nations, and US-American Native Indian cultures and literatures was prepared by Angeline O'Neill (The University of Notre Dame, Australia) and Albert Braz (University of Alberta, Canada) and published in 2011:
O'Neill, Angeline; and Braz, Albert. "Bibliography of Scholarship on Indigenous Literatures and Cultures." CLCWeb: Comparative Literature and Culture 13.2 (2011): <http://dx.doi.org/10.7771/1481-4374.1749>