Several databases can be good beginning points with introductory overview essays on your topic and some include bibliographies of the principal recommended secondary and primary sources. These can be good starting points for thinking about your topic and identifying the key recommended works.
Reference sources published by Oxford University Press. Dictionaries, subject reference works, language reference sources and key titles in the Oxford Companion series. Some examples are: the Oxford-Duden German dictionaries, A Dictionary of Psychology, A Dictionary of Zoology, The Oxford Dictionary of Idioms, A Dictionary of Writers and Their Works, Oxford Companion to Western Art, Oxford Companion to U.S.History, Oxford Companion to Music, Oxford Companion to Philosophy, and The Encyclopedia of Popular Music, etc.
Concise overviews of hundreds of diverse topics from the abstract (Knowledge) to the concrete (Coral Reefs), written by experts in the field who combine facts, analysis, new ideas, and enthusiasm to make challenging topics highly readable.
Subject specific encyclopedias and historical dictionaries are useful points of departure for your research. Some of the titles below can be accessed online.
Good question -- here are a few reasons:
Companions are books authored and edited by scholarly writers that give background information about topics. This background can further your understanding of the issues and/or complexities surrounding your topic and the time period when it takes place.
You can find "companions" on many topics. Just do a keyword search in Library Catalog for the word "companion" and the word(s) for your topic to see if one has been published. Below are some examples of the variety of companions you might encounter - from the broad to the narrow in scope.