1 Lucas: {over walkie-talkie} We have to go look for Will! The more people the more chance of finding him!
2 Mike: Okay, fine. Get Dustin. I’ll see you on Mirkwood in a few minutes.
3 Lucas: Bring your D&D rule books.
3 Mike: What for?
4 Lucas: If we get into trouble, we can attack people with them. There’s no way they’ll get back up after being whacked with the Dungeon Masters Guide!

I've mentioned before that the original 1979 Dungeon Masters Guide, while a massive and weighty tome worthy of being used as an improvised weapon, had no apostrophe in its title. The second edition version, released in 1989, was renamed the Dungeon Master Guide, while the subsequent 3rd, 4th, and 5th editions have all been called the Dungeon Master's Guide, with an apostrophe indicating it's a guide for a single Dungeon Master.

I'm eagerly anticipating the future Dungeon Masters' Guide - the one for all the Dungeon Masters.