There are two types of books for D&D: rulebooks, and modules. Rulebooks like _Player’s Handbook_, _Dungeon Master’s Guide_, and _Monster Manual_ basically **are** the game; without at least the core rules, you have to make up how everything works yourself and you’re not really playing “Dungeons & Dragons” per se. (Instead, you’re playing in a homebrew RPG system, or playing freeform, that is, without rules. Both are entirely valid and we take questions about these activities, but if you call them “Dungeons & Dragons” people will get confused.)

There are core rulebooks (e.g. _Player’s Handbook_, _Dungeon Master’s Guide_, and _Monster Manual_) and supplements (e.g. _Tasha’s Cauldron of Everything_). The core books are usually the bare minimum; supplements are optional. (It is possible to play without the _Dungeon Master’s Guide_ or _Monster Manual_, but it can be hard, especially if you are new.)

The other books, modules, contain premade adventures like _Tyranny of Dragons_ or _Curse of Strahd_. You need them to play those adventures, but you don’t need to play those adventures. You can make up your own adventure, even your own world, and just play with the rulebooks. That is definitely still D&D—in fact, traditionally, it’s the default and norm.