The Invincible, Solaris, and Eden all deal with the question "what if aliens are truly alien, and we cannot find any points of contact with them?"
Even his lighter work (Cyberiad, Ijon Tichy) is very insightful
And outside of Sci-Fi he was still an absolute powerhouse. "Hospital of Transfiguration" is an absolutely haunting novel that you will keep thinking about for months later if not years, and "Journals found in a bathtub" is on a different level.