KEEPER'S COMPENDIUM features information on strange places (Mu, Hyperborea, Kadath, R'Lyeh), as well as the usual strange races (Mi-Go, ghouls, Deep Ones, etc). Plus description of various secret societies (Witch Cults, Starry Wisdom, Golden Dawn) that can aid or hinder the characters and their various agendas; description of occult and mythos tomes; Forbidden Lore, including editions in various languages; rules for translated works, and more!
Basic Premise: A general reference guide for people who run the Call of Cthulhu RPG.
I love how well the various CoC books work together regardless of edition. Seriously, there's next to no differences in rules there. This is an older edition of the Compendium, but because the game is much more about the role-play (rather than the roll-play), the amount of "fluff" over "crunch" in this book is valuable. Keepers have to know how the world/story works far more than they need extra rules for how to roll the dice. This book is helpful for all of that.