Eeep! Where do I begin? Everyone had some really good recs, especially The Giver (one of my favorites) Fahrenheit 451, and the Chronicles of Prydain.
I mostly read MG, but I’ve found some with great themes. If you’re interested, they are:
The True Confessions of Charlotte Doyle and Crispin: Cross of Lead (by Avi)
The Bronze Bow (Elizabeth George Speare; actually, pretty much any of her historical fiction has strong themes. Bring tissue, cause this one’s powerful).
Medallion (Dawn L. Watkins; it’s published by BJU Press, so I don’t think they have an ebook for it…? I only found the paperback copy on Amazon.)
When You Reach Me (by Rebecca Stead)
Walk Two Moons (Sharon Creech)
Return to Christmas (Chris Heimerdinger)
Catherine Called Birdy (Karen Cushman)
And if you are looking for more of a YA title, I have this one:
A Separate Peace (by John Knowles *disclaimer: has some language*).
I hope this is helpful; if you want more, please don’t hesitate to ask! 🙂
We are all apprentices in a craft where no one ever becomes a master. — Ernest Hemingway