(Vonnegut has other good scifi, i quite like The Sirens of Titan)
i must recommend The Forever War
Two books I read within the past six months or so and I fully endorse that recommendation. I didn't really appreciate Sirens until I had finished and digested it all, great novel though. The Forever War really stood out for me as a realistic example of time dilation in an interstellar war, at least from the perspective of the "soldier."
All the authors I would have otherwise recommended have already been mentioned. Starship Troopers is another recent read that I thoroughly enjoyed.
EDIT: Came to me this morning, I really liked The Dosadi Experiment by Frank Herbert. The planet in question reminded me a bit of Ishual