One book for an 11-year-old wanting to GM?
For me there's no question the best book is Return of the Lazy Dungeon Master by Sly Flourish, aka Mike Shea. The book takes a minimalist approach of only prepping what you need, understanding that you'll have to improvise so prepping to improvise, and modular prep so there's a minimum of wasted effort. It's easily the most practical and immediately useful how-to guide out there right now. Mike supports the book with regular YouTube videos and explainers if something doesn't make sense. He also does regular videos with GM advice. It's written in a very straightforward, matter of fact style that doesn't waste time or words.
For monsters, I'd suggest Sly Flourish's Forge of Foes. It's also a how-to guide that helps you design your own monsters and create encounters for your D&D games. There's a chart of quick stats in the back that you can just print out and use as your Monster Manual.