Title: the Forever War
Author: Joe W. Haldeman
Haldeman, a veteran of Vietnam, wrote the Forever War about Vietnam. However, it could just as easily be about any war in the history of mankind, and is really about the damage that war does, and its futility. The book won a Hugo and Nebula, and deserves both.
The Forever War chronicles the military career of William Mandella, the son of 1970's hippies who is drafted into the military in the mid 90's to fight a war against an unknown alien race. Because of the effects of space travel, and dumb luck, he ends up surviving the whole ~1100 year war, even though he only ages a couple years of subjective time.
I won't give away the ending, except to say that the effects of space travel, and changes in society over the eyars create much the same effect for Mandella that most soldiers report experiencing during most wars, of being disconnected with the world they've returned to. The world has changed so much by the time that the war ends, that, were I in Mandella's shoes, and the war ended slightly differently, I probably would have committed suicide.
Overall however, this is an amazing book, one that even people who don't like sci-fi that much should find something of interest in. the Forever War should be available at most bookstores, just make sure you get the author's preffered edition, which contains the original manuscript.