Among the sci-fi community, Joe Haldman's book The Forever War is considered one of the three founding novels of the military science fiction genre (the other two are Starship Troopers and Ender's Game). I recently read it, and I was pleasantly surprised. It is one of the best science fiction novels I have ever read.
Here is the blurb from the back cover of The Forever War.
This book is a shining example of everything a science fiction novel should be. Joe Haldman takes scientific accuracy seriously, and his story reflects this. Of course, the typical clarketech aspects are present (such as FTL travel), but this tech is depicted in a consistent and convincing fashion. Haldman manages to convincingly depict a universe where, due to time dilation, a single war is fought across centuries by hundreds of generations of humans. Ultimately, this allows Haldeman to examine the concept of transhumanism as the main character gets to witness humanity's evolution across the millennia.The Earth's leaders have drawn a line in the interstellar sand--despite the fact that the fierce alien enemy they would oppose is inscrutable, unconquerable, and very far away. A reluctant conscript drafted into an elite military unit, Private William Mandellla, has been propelled through space and time to fight in the distant thousand-year conflict, to perform his duties and do whatever it takes to survive the ordeal and return home. But "home" may be even more terrifying than battle, because, thanks to the time dilation caused by space travel, mandella is aging months while the Earth he left behind is aging centuries...
The Forever War's main theme is alienation, as the protagonist finds himself in an unfamiliar human world hundreds of years beyond his own. This is based on Haldeman's own experiences as a soldier returning from Vietnam to an unfriendly nation. Although Haldeman chose such serious subject matter, he deals with it in a witty and lighthearted fashion. Today's overly serious and moody science fiction would do well to study his example (that's right, Battlestar Galactica, I'm looking at YOU.) The Forever War is a book that's fun to read, but also has profound insights into alienation, transhumanism and evolution. It is a thought provoking read, and I highly recommend it.