Myself. I have over twenty years of GM experience and most books don't explain the primacy of a GM.
Robin's Laws of Good Game Mastering: explains the variety of ttrpg playstyles and how a GM can design campaigns that focus on them.
World Builder's Guidebook: explains how to easily create unique settings using random tables.
Play Unsafe: explains how to incorporate improvisational techniques into creating and running ttrpg campaigns.
Master of the Game: Gary Gygax explains how to become an expert GM.
Honestly.... if the kid has played a few times... hand the kid the DM's guide (optional), monster manual and a sheet of graph paper for the dungeon.
Best way to learn is to do it. Giving the kid some how to guide to read through will probably just dampen the enthusiasm and make it seem like there is a right way and a wrong way.
I first thought these two ideas myself.Matt Coville first, say, 8 YouTube videos.
My brothers and I started with the 83 Red Box and that had one of the best "learn to DM" tutorials out there. Solo adventure --> premade dungeon level --> you stock the next level --> you draw and stock the final level.I first thought these two ideas myself.
How did I learn to DM? I played a few/several times and then saw some of the other kids being the DM and tried it myself. I'm sure most was terrible at first. Maybe another tip might be to play a few times, make a small dungeon with you there to help ask some guiding questions, then go have fun.
I recall being 11ish and not really going to read a book about playing a game. I think I read most of the PHB and looked at a lot of pictures in the MM, but never read the whole DMG to this day in any edition. Maybe aggregate over the years by reading lots of portions as I needed them, but not to just sit there for enjoyment.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.