Ender's Game - Orson Scott Card, a less mature but stellar read.
By less mature, I assume you mean not appealing to anybody over the age of 12. I read Card's book when I was in sixth grade and found it to be a boring and poorly written story about stupid kids playing war games in zero gee and essentially a script for the Disney Channel. If you really want some good Sci-Fi with galaxy spanning high concepts that test your level of comprehension, try "A Fire Upon the Deep" by Verner Vinge.
If you're in an ugly mood and want to read some dark yet brilliant shorts, try the 'Angry Candy' anthology by Harlan Ellison.
Don't rule out graphic novels either. They've kind of fallen out of favor lately, but anything by Allan Moore like the superb 'Watchmen' is worth your time.
I recently read 'Dancing Naked in the Mind field' by Nobel prize winner Kary Mullis, and found it interesting as hell. Most geeks would dream of having the I.Q. of Mullis, and the eccentricity.