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.
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 ACORN 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.