Books

All offical rules, races & classes, monsters, and spells can be found in the many books published by Wizards of the Coast. These books provide information on gameplay and the larger world.
Some books even have pre-made stories Dunegon Masters (DMs) can use to run a session or campaign. Though there are several published books, there are three main books that can benefit any player:

The Player's Handbook - creating a character, combat rules, list of spells

The Dunegon Master's Guide - how to run D&D adventures

The Monster Manual - guide to monster stats and mosnter lore

Those these are the official materials, D&D is an open place to create. Play the characters you want with the people you want. While Wizard of the Coast made an vast and entertaining game, parties
and players can take their own route. Creative DMs can create their own world with their own creatures and own forms of magic.