It's such a terrific book, I think I first read it when I was like 10 or 11?
If you want another us vs. aliens, that was a really fun plot mechanism (hint: interstellar travel back and forth takes a _long_ time), check out The Forever War by Joe Haldeman, also very much a classic of the genre.
Thanks for the recommendation, I just finished Starship Troopers last night and I just purchased "The Forever War". I will be starting it tonight!
The Forever War