Honestly, the more D&D (or other applicable TTRPG) sourcebooks you have, the more prepared you have the capacity to be. This being said, none are absolutely required for this class. If you're playing D&D and looking for a place to start, I would absolutely recommend the Player's Handbook and the Dungeon Master's Guide.

Dungeons and Dragons games vary widely on content, and can go from zero mature content to vastly mature depending on the game and group. While running or discussing any games on Outschool, I will be staying far from all mature stuff. It does involve violence, as D&D is known for combat as well as role-playing, but I won't be using any grotesque, visceral descriptions of things.