Reference sources are helpful resources to learn more about your topic or research subject. These types of sources aren't meant to be read cover to cover; rather you refer to them for specific pieces of information about your subject. Reference sources can provide information such as topic overview, background information, definitions of associated terms, related topics, and bibliographic information for further reading.
There are a variety of types of reference sources, and these include encyclopedias, atlases, dictionaries, directories, handbooks, almanacs, yearbooks, and indexes.