I'm not taking issue with the other recommendations, they're excellent, just adding this one, from the non-humorous hard sci-fi side of Lem: The Invincible. Rea

I'm not taking issue with the other recommendations, they're excellent, just adding this one, from the non-humorous hard sci-fi side of Lem: The Invincible. Really short, really tight, some very hard sci-fi concepts being explored, still works as a story. A bit spooky.