Starship Troopers by Robert Heinlein
It's nothing like the movie. It is about this kid, Juan "Johnnie" Rico who enlists in the Mobile Infantry and fights aliens; But I've always felt the book was about the ideals behind-the-scenes. (Man's "right" to live and conquer, ethics of war, citizenship and civic duty, etc.)
In short, it's a straightforwad action/adventure novel with a philosophical backdrop.
EDIT: Then again, that book is kind of controversial. Also from Heinlein, I'd recommend The Moon is a Harsh Mistress. It's about the revolution of citizens on the moon (Luna), led by an aging professor, a fiery female political agitator, a computer technician, and their BFF, "Mike" -- a childish supercomputer. It's fun and witty and full of freedom and sci fi.