The Gem Cutter's Guide

From the Introduction:

This book was written to introduce novices to becoming a Game Master. It doesn't explain an real rules, but instead focuses on show the Game Master how to handle situations and set her stage for adventures.

This book is divided into five main sections. Every Game Master should know and understand the material in the Basic Storytelling and Non-Player Character chapters. The others can come with more experience.

In truth, however, most of the book will seem like common sense. It's surprising, though, to learn what every good Game Master forgets from time to time.