Check out the list of Newberry Honor and Newberry Award books. Even today I will happily sit down with one of these--shorter, simpler prose combined with incredibly powerful stories.
Since you asked for sci-fi in particular, here are a few Newberry suggestions that may suit:
The Giver by Lois Lowry (Houghton)
A Wrinkle in Time by Madeleine L'Engle (Farrar
The High King by Lloyd Alexander (Holt)
Full list can be found here:
Check them out, you wont be sorry.