There are lots of roleplaying games to play, Dungeons and Dragons being the most popular. I am going to say the main books needed for it:
Player’s handbook: This will tell you all the basic rules for the game and what special powers your character gains because of their race and class.
Dungeon Masters guide: You will need this if you want to be a Dungeon Master. It will tell you how to run a good game and high-powered character options.
Monster manual(1, 2, 3, 4, 5 and Fiend Folio): These have monsters to run the game with, from dragons, dire animals, evil undead and werewolves.
Do YOU have these books at home?