The Forever War (1974) by Joe Haldeman is a kick in the gut, followed by being drawn and quartered. This books grabs you by the throat and demands your full attention, sir, yes sir. A relatively short read by modern standards, it makes use of every word, no padding necessary.
Relativity is the name of the game, and as soldiers go off to fight a future war, the world they left behind becomes increasing strange, increasingly alien, until there is literally no difference between the aliens that they are fighting and the aliens that they defend. Everyone becomes a man out of time, living hundreds of years beyond any time known.
While this may be a war story, in no way does this story glorify war. As time and technology creep forward, so do the horrors of war, men in mechanical suits treated as disposable as any other piece of war equipment, their lives enslaved to an endless war. A forever war.
Is this book really that good? Yes, it really is that good.