Before jumping into your search, it is always a good idea to visit reference sources as a means of learning about your topic at a general level. This will not only help you gain a stronger knowledge about your topic, historical figure or event, but it also help you identify important keywords and concept that you will need to know when you start searching for books and journal articles.
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The following is a small sample of the many History-specific Encyclopedias and Dictionaries located in the LLC's Reference Room (REF):
The Oxford Dictionary of World Religions by 
Berkshire Encyclopedia of World History by 
A Dictionary of the Roman Empire by 
A Guide to the Ancient World by 
Encyclopedia of Ancient Egypt by 
Encyclopedia of Ancient Greece by 
Concise Encyclopaedia of Islam by 
Dictionary of the Middle Ages by 
Medieval France by 
The Oxford Encyclopedia of the Reformation by The Cambridge Encyclopedia of Russia by 
The Encyclopedia of World War I by 
The Oxford Companion to World War II by 
Encyclopedia of the Holocaust by 
Encyclopedia of Latin American History and CultureAdams Dictionary of American History by Dictionnaire des femmes célèbres by 