“A companion is not a dictionary or an encyclopedia or a literature review. Unlike a dictionary, it is not primarily intended to provide an explication or a history of technical concepts; it is meant to offer substantive commentary on the works pursued in the relevant field of study. Unlike an encyclopedia, it is not committed to the systematic perspective of the office record; it is deigned to be a practical guide for someone who wants to find their way through the relevant field. Unlike a literature review, it is not directed only at professionals in the area; it is also written with a view to those who come fresh and unseasoned to the topics discussed”
Quoted from the Introduction chapter of this book.