Joe Haldeman - The Forever War

A taut sci-fi/military novel that doesn't do much wrong, written in the shadow of the Vietnam war and unmistakeably, but never distractingly allegorical in its depiction of the struggles of its soldier protagonists to adjust to the relativistic effects of their campaigns, where time passes vastly quicker on earth than it does for them subjectively, decades passing on earth while they experience only weeks or months in training and battle on alien soil.