I'm back with a couple more!
The Forever War by Joe Haldeman. It's about a war fought in distant star systems where the travel to and from engagements at relativistic speeds causes the soldiers to sometimes arrive with hopelessly outdated intel and technology, or other times to massively outclass their enemy, but always to return home to societies that are completely alien to them.
The Man Who Folded Himself by David Gerrold. Time travel shenanigans. I like to call it "The Man Who Fucked Himself".
I really need to read more of her stuff, because I've only read that and Left Hand of Darkness, and tbh she's one of my favourite authors on the strength of those two alone.