Dungeon Master's Guide
The Dungeon Master's Guide, abbreviated as DMG, is one of the three volumes that Wizards insists on republishing and revising every edition of Dungeons & Dragons. One of them, the Monster Manual is also intended for the Dungeon Master but is intentionally a seperate tome because otherwise it would double the size of the DMG. It basically tries to teach you how to be a good Dungeon Master DM despite the fact that good DMing comes from experience and not from reading the Idiot's Guide to DMing.
The book also contains lists of magical items, rules of their creation and generation, additional rules that players don't need to be aware of, and lots of useful tables.
DMing can become quite a complicated task without a DMG, so it is advisable to have one in one form or another.