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 the 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.