Background sources are books, encyclopedias, almanacs, reference books, etc. that contain condensed information about a subject. They are an excellent starting point when you are new to a topic and can help with finding quick definitions or the basics relating to a broader topic, finding key people, and locating other readings.
Some of these resources may have an older publication date but are seminal reference works, or have annual updates if it an electronic subscription.
An online reference library comprised of reference books including encyclopedias, dictionaries, thesauri and books of quotations, etc.