The Forever War
The Forever War – Joe Haldeman
Good sci-fi about a (too long) war in space with another race. It describes things like how the world changes (in waves) and how people in the war are different than civilians (also now the case). Fun idea about homo vs heterosexuality.
Not the best book ever for me, but I think it might be one of the first to use time-dilation in this way to let the characters move forward through time so elegantly.