An excellent resource for the history of the theater, the theater as literature, and theater as it is culturally situated.
The MLA International Bibliography, produced by the Modern Language Association, consists of bibliographic records pertaining to literature, language, linguistics, and folklore. The MLA International Bibliography provides access to scholarly research in over 4,400 journals and series. It also covers relevant monographs, working papers, proceedings, bibliographies, and other formats. major article indexes.
Coverage: worldwide, ca. 1850-2005. Concentrates on drama as literature, rather than as performance. The best place to look for authoritative biographical information on lesser known playwrights from around the world. There are also extensive entries on individual works (e.g. Zong Fuxian's "In a Land of Silence").
Three volume history of British theater starting around the time of Shakespeare and continuing forward. An excellent source for background information and contextualization of British theater. Online version (at link) lacks volume 3. Paper edition at call number is complete.