I would highly recommend:
Ender's Game (and ensuing sequels). Some of the best stuff I've ever read, as a few have noted.
The Dresden Files by Jim Butcher (the first two books are slow, but they are quick reads and if you get past them, the series REALLY pays off).
The first trilogy of S.M. Stirling's "Dies the Fire" series. Pretty cool post-apocalyptic stuff.
"You gotta love livin', cause dying is a pain in the ass."