The Chronicles of Narnia by C.S. Lewis– even better as an adult, especially if you’re interested in Christianity. You’ll be able to appreciate lessons you couldn’t even see were there as a child.
Ender series/Ender’s Shadow series by Orson Scott Card– awesome scifi series that explores a lot of ethics and philosophy. Brainlets might find it boring after Ender’s Game, but Children of the Mind is one of the most satisfying conclusions I’ve ever read.
Hard to be a God by Arkady and Boris Strugatsky– same writers as Roadside Picnic; the story of a man in the future who is studying a world similar to medieval Earth. Well-paced mix of science fiction and fantasy.
METRO2033, 2034, 2035 by Dmitry Glukhovsky– very personal, “human” story of a young man living with other survivors in the Russian metro stations after the end of the world. The three books have very different vibes from each other.
Roadside Picnic by Arkady and Boris Strugatsky– short sci-fi story that inspired S.T.A.L.K.E.R. Dudes in gasmasks going treasure-hunting in spooky radiated wastelands.
The Screwtape Letters/Screwtape Proposes a Toast— demon instructing his students on how to be subversive
The Space Trilogy by C.S. Lewis– Tolkien in space. Incredible sci-fi. Explicitly Christian, but anyone should be able to appreciate the pictures Lewis paints of alien worlds.
The Things That Are Not there by C.J. Henderson– Lovecraftian nightmares vs. hardboiled detective. Not the usual utter hopelessness of pure-Lovecraft. Whether that’s desirable or not is up to the reader.
The Witcher Series by Andrezej Sapkowski– (fantasy- elves, dwarves, monsters, etc.) I admit I could never get much into the games. They played very nicely, but I didn’t know what the hell was going on. Thank Melitele that there’s extremely well-written, engrossing books to provide me with backstory. Start with The Last Wish and Sword of Destiny before moving to the main books.
The timeless work of Peter Chimaera— the master who first inspired me to start writing.
My Immortal by Tara Gilesbie— better than the last 3 Harry Potter books in every way.
Nonfiction, Classics, etc.
The Art of War— I just read it because I’m a weeb, but there’s wisdom in there.
The Book of Five Rings— I just read it because I’m a weeb, but there’s wisdom in there.
The Prince— the only good thing about the last few Harry Potter books was Dumbledore’s convoluted scheming. “The Prince” is the Bible of such plotting and I can’t recommend it enough. The copy I read included the Life of Castruccio Castracani, a short biography I would recommend as well.
Romance of the Three Kingdoms— Cao Cao did nothing wrong.