The Game Masters Handbook deals with the ways and means of doing things in Quest. It covers the mechanics of fighting, spell casting, giving out experience, and so on. It also contains information on creating a world for role-playing, populating it with people and creatures, and incorporating technology and magic into your Quest campaign.
It is assumed that before reading this, the Game Master has at least a passing familiarity with the material in the Quest Player's Handbook. If not, many of the terms used here will be unfamiliar.