Reference sources are available in general or subject specifics contexts and in print or online formats. You need to evaluate each source and determine which are helpful to you and appropriate for you to use.
Examples:
Encyclopedias
Dictionaries
Almanacs
Glossaries
Handbooks
Atlases
Bibliographies
Indexes
Reference materials are good starting points for research projects because they can provide:
background or introductory information on a topic
terminology or additional keywords to use in further searches
Award-winning biographies and full-text articles from hundreds of periodicals. Search for people based on one or more personal facts such as birth and death year, nationality, ethnicity, occupation or gender, etc.
provides full text for nearly 2,050 general reference publications with full text information dating as far back as 1975 and includes more than 350 full text reference books, 82,968 biographies, 90,915 primary source documents, and an Image Collection of 107,135 photos, maps and flags.
The Oxford Companion to American Theatre by 
The Oxford Companion to Theatre and Performance by