Encyclopedias help you figure out the who, what & where of a topic. Knowing the who, what & where of a topic is the first step in researching a topic and discovering the "how" and "why". Encyclopedias and dictionaries can provide background information and can be used to identify key people, events and dates which can then be used as search terms for finding additional information. Many also provide a jumpstart on your research by listing related readings.
Major encyclopedia: Oxford Research Encyclopedia: American History
These books provide a survey of the state of research about a topic or region, historiography or concise overviews.