Encyclopedias are books that contain factual information about many different subjects. They are typically organized alphabetically by the name of the subject or topic. The amount of information provided in encyclopedias can vary from a few paragraphs to several pages.
There are two types of encyclopedias -- general and subject-specific. General encyclopedias provide overviews on a variety of topics. Subject-specific encyclopedias contain many entries that all focus on one field of study. Typically, general encyclopedias are published as multiple-volume sets whereas subject-specific encyclopedia may be made up of one volume or multiple-volume sets.

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