Once and Future King by T. H. White. Collins, 1958.
I read Once and Future King when I was 14, an age I qualify as older tween. There were two things about the book that came from opposite ends of the age spectrum that made me want to read it. Two reasons that I believe serve as an example of the mental turmoil older tweens are going through about trying to figure out their place in the world. I wanted to read Once and Future King because one of my favorite Disney movies as a kid had been The Sword and the Stone, and because it looked like a book grown-ups would read, so I was totally all over it.
This was one of the first books that changed my life. It shifted the way I looked at the world. I think a big part of that is the presence of mythology running through the narrative. White updated the story of Arthur to have it be contemporary to his time, to make it relatable, which is good since there was no way I was going to read Sir Malory’s version at 14 (I barely got through it in college).
The importance of myth and recurring themes in narrative to tweens is that they can then set their own lives to a narrative. You don’t feel like such a loser when you’ve read about how even King Arthur was young once, and he had to deal with Merlin being cranky and turning him into a bunch of animals so he could learn about life.
So yeah, I think basically every well read 13 and 14 year old needs a copy of Once and Future King.