Dungeon Mastering can be easy, but to do it well, you will have to know all the fundamentals. This core book from Wizards of the Coast is designed precisely for that. It is simply invaluable if you take DMing seriously
 
It covers, game mechanics, world-building and a whole slew of items and roll charts to help you build your campaign and sessions. This book is both comprehensive and essential.
 
For New DM's - this book will get you up to speed quickly, providing more confidence in preparing and running your games.
 
For DM's who wants to level up - This is a Must Have! The vast amount of info and mechanics will provide you so much more in developing your encounters, plot lines and NPC portrayal. This is especially helpful if you are Homebrewing your campaign and sessions.
 
For Experienced DM's - This is a great quick reference for that odd situation you are facing at the table. Also, the list of Magic items in this will benefit your campaign and delight your players.
For Homebrew campaigns - This will deepen your campaign considerably, aiding in developing better plot lines and layering your encounters.
 
This book is NOT FOR PLAYERS.