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Slaying the Dragon: A Secret History of Dungeons & Dragons by Ben Riggs
So You Want to Be a Game Master: Everything You Need to Start Your Tabletop Adventure for Dungeons and Dragons, Pathfinder, and Other Systems by Justin Alexander
The Monsters Know What They're Doing: Combat Tactics for Dungeon Masters by Keith Ammann
The Ultimate RPG Game Master's Guide: Advice and Tools to Help You Run Your Best Game Ever! by James D'Amato