Overview sources are great tools with which to begin a research project. They may be helpful for obtaining a general overview of a subject, and they may also help in the narrowing of research topics and in placing a topic in the proper context.
Some sources, such as subject dictionaries, will provide an introduction to the specific language of a subject or discipline. Sources may cover a fairly broad subject area or a particular aspect or topic in the discipline.
Entries will often include a bibliography, and they may reference additional related books, articles, documents, and other useful sources.