@nixie As for fantasy... I'll second
The Princess Bride and
The Mouse that Roared.
I'll also add
Mistress Masham's Repose by T. H. White. It's about a little girl in post WW2 England who discovers something fantastic. The themes are courage, fidelity, and imperialism versus self reliance.
Other books that go much deeper than their seemingly young reader status are C. S. Lewis' Chronicles of Narnia series. Ostensibly about children and tweens adventuring in a fantastic world, the real themes are the natures of man and God, fidelity, hope, forgiveness, sacrifice, courage, and acceptance.
The Lion, the Witch, and the Wardrobe is the first (and an excellent book), but my favorites are
The Horse and his Boy,
The Voyage of the Dawn Treader,
Prince Caspian and
The Last Battle.
If you want to explore the pulp age of sci-fi, then Edgar Rice Burroughs' series on Mars. It's not too sophisticated... let me be kind and say it's uncomplicated. The hero is righteous and his cause is just. The villains are evil and debauched. The damsel is in distress... in fact she's always a beautiful princess. The odds are long.... The themes and the motivations and behavior of it's characters really shed light upon the serialized story telling in the popular magazines of the post-Victorian age. The first book,
A Princess of Mars sets the stage. The next four books follow right in line. By then, any magic or sense of wonder is gone. I've read them all, but the last six are easily forgotten and dismissed. My personal favorite is the fifth book,
The Chessmen of Mars, which you can guess.... human chess pieces that play to the death. (I suggest a new strategy R2, let the wookie win.)
Another book that is massively uncomplicated and yet is enjoyable is
Legend by David Gemmell. He takes all the best aspects of the Battle of Thermopylae, Conan the Barbarian, and John Wayne movies and brings them together.
And the last non-fiction I read was
The Black Count: Glory, Revolution, Betrayal, and the Real Count of Monte Cristo by Tom Reiss. I whole heartedly recommend this. Beginning with
On Cherry Street (my first grade primer) and continuing through
Interpreting the Bible by A. B. Mickelson,
The Count of Monte Cristo is my favorite book that I was required to read in school. Discovering this biography of the real count was uplifting and a bit unsettling. I've always thought that Alexandre Dumas had a superb imagination, but it turns out that he just had a larger than life father from whom to take stories. I cannot imagine that five years will pass before Hollywood turns this into a major film starring Michael B. Jordan.