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Encyclopedias are reference books that provide summaries of information across a range of alphabetized topics. Not everything in an encyclopedia will be relevant to your topic; you will have to find your topic within the encyclopedia by its first letter (for example, "Ancient Greece" could be found under A, or also be a subtopic of "Greece," which would be found under G).
Encyclopedias are particularly useful when you need to provide a range of information about a broad topic, rather than a "deep dive" study into a particular topic.