Dungeon Master's Guide
The Dungeon Master's Guide, abbreviated as DMG, is the text which lays out such rules as the game referee should know and the players should accept. As the title suggests, DMG comes out of Dungeons and Dragons - specifically, 1980ish AD&D, to which the sideline caught up, in 1983 under Mentzer. Some say, and we would agree, that good DMing comes from experience more than from reading the Idiot's Guide to DMing. But we were all idiots once.
Among what the DM should know and players shouldn't are the vital statistics of magical-items and monsters. TSR quickly found that monsters had minds of their own, so split them into their own separate tome. Although some monster data had to be dealt with, here, too.
The DMG has become one of the three volumes that TSR > Wizards > Hasbro insists on republishing and revising every edition, alongside the Players Manual which lays out the rules everyone has to agree upon, and that Monster Manual. DMing can become quite a complicated task without a DMG, so it is advisable to have one in one form or another.
The first Advanced Dungeons & Dragons version was bonkers. Gary Gygax wrote it without much editorial intervention; it is notable for historical references few of the 11 year old kids whose parents bought the thing for them would even know, like Rasputin's 18 Constitution (their parents, pre-Internet, weren't much able to help, either). The later editions focus more on being useful utilities for their versions of D&D.
Generation X grew to love this first edition exactly because Gygax didn't talk down to them, and instead pointed them to an extended vocabulary and those historical references. Appendix N was a particular high point. We weren't getting any of this shit in our English classes. Bless you Gary for all you have done to inspire us.