The Forever War (Joe Haldeman)

This is a classic, from 1974, but it is still compelling. The central character is one of the first drafted into interstellar war in the early 2005, and with all the relativistic time-dilation effects, he makes it to the end of the war in 3143. Along the way technology and human nature both change, but he remains much the same, giving the reader a consistent viewpoint with which to view time going by. It is a combination of war story, with high-casualty battles and high-tech weaponry (with few false steps to remind us that it was written before ubiquitous computers and the Internet), and social commentary. I enjoyed it a lot.