A really great way to start researching a new topic is by getting an overview or in-depth background information. One of the (many) great things about reference works in theater is that they are written by scholars in the field and often include references to additional secondary and primary sources.
While the list of titles on this page includes a selection of both contemporary and older reference works, older works may be helpful with historical research, and/or may give you an indication of a particular call number range that could be relevant to your research. (Note: Call numbers allow books to be shelved by subject. So if you find one book about Broadway musicals, browse that section to find additional, related works.)