Underworld and Wilderness Adventures

The actual method of how to play D&D is contained in this volume. Men and Magic and Monsters and Treasure describe the things in D&D and how to use them, but the actual explanation of what to do in the game is written in Underworld & Wilderness Adventures.

And there's not a lot in there. The first section describes dungeons and how to build one followed by a brief discussion on how to explore it, with a couple of short asides on how to actually use some of the items detailed in the equipment list.* This is followed by a discussion of combat procedures in the dungeon, which explicitly directs readers to Chainmail for an in depth explanation, or they can use the very sparse charts from the alternate combat system in volume I.

The next section is Wilderness and it basically directs the reader to Outdoor Survival for the actual exploration rules and a map on which to play. A short discussion on converting the Outdoor Survival map for D&D play follows, and the rest of the book is taken up by combat procedures on land, aerial and naval.

What's striking is what's missing. There is no discussion of roleplaying, no mention of town adventures, of chats with NPCs, no instructions for the creation of a campaign or set up for heroic quests. D&D is primarily and explicitly only about combat, on a 1:1 figure scale in a fantasy setting either in the underworld or overworld, with monsters and heroes.  What people actually consider an RPG to be is completely absent.

Maybe all of that is contained in the supplements and Strategic Review magazine, but then that would mean that to canonically play D&D as an RPG requires a whole host of sources beyond the basic 3 booklet set.  Everything I've mentioned that was missing is actually in AD&D and the later Basic sets.

*which I found way more helpful than most other D&D source books since this information is not restated anywhere else until the Mentzer red book.