C.S. Lewis is my favorite author so I'd highly recommend his space trilogy (Out of the Silent Planet, Perelandra, and That Hideous Strength) as well as his final novel Till We Have Faces. If you like Lewis and want to get a bit deeper into Faerie, try some of George MacDonald's works: The Golden Key, The Light Princess, The Princess and the Goblin, The Princess and Curdie, At the Back of the North Wind, and Phantastes are worth checking out. I also recommend The Neverending Story by Michael Ende—it's much better and richer than the film (sorry). Ursula K. LeGuin's Earthsea books are also worth checking out, though I've only read the first (A Wizard of Earthsea). Oddly enough, I'm also a huge fan of Garth Nix's Abhorsen quartet (it used to be a trilogy): Sabriel, Lirael, Abhorsen, and Clariel. If you like those you may also want to check out Across the Wall: A Tale of the Abhorsen and other stories, a collection of stories by Nix that gives a couple of side stories. Roger Zelazny's work is also very well done and entertaining: try A Night in the Lonesome October and Bring Me the Head of Prince Charming. Finally, for classic sci-fi, try Tunnel in the Sky and Orphans of the Sky, both by Robert Heinlein, and Stanislaw Lem's Cyberiad.
So, to recap:
C.S. Lewis:
- Out of the Silent Planet
- Perelandra
- That Hideous Strength
- Till We Have Faces
George MacDonald:
- The Golden Key
- The Light Princess
- The Princess and the Goblin
- The Princess and Curdie
- At the Back of the North Wind
- Phantastes
Michael Ende:
- The Neverending Story
Ursula K. LeGuin:
- A Wizard of Earthsea
Garth Nix:
- Sabriel
- Lirael
- Across the Wall: A Tale of the Abhorsen and Other Stories
- Abhorsen
- Clariel
Roger Zelazny:
- A Night in the Lonesome October
- Bring Me the Head of Prince Charming
Robert Heinlein:
- Tunnel in the Sky
- Orphans of the Sky
Stanislaw Lem:
- The Cyberiad
Enjoy!