Dictionaries, encyclopedias, and handbooks, or tertiary sources, are often overlooked sources of scholarly knowledge. If you want a great definition of a theory that goes beyond Wikipedia, I highly recommend some of these titles held by UVic Libraries.
Tertiary source: "A written work, such as a chapter in a textbook or entry in a reference book, based entirely on secondary sources, rather than on original research involving primary documents." (The Online Dictionary for Library and Information Science).