A second vote for Robert Heinlein. I'm not a big fan of science fiction but there's no better guilt-free genre for philosophy and social commentary.
A third vote
My elementary school library had copies of several of his "juvenile" works. Books like "Have Space Suit - Will Travel", "The Star Beast", "Glory Road", "Space Cadet", and "Tunnel in the Sky". These led me to his more adult books, before I was aware of the distinction...
I still credit/blame a lot of my non-standard moral compass to probably-far-too-early readings of things like "Stranger in a Strange Land", "Farnham's Freehold", and "The Moon is a Harsh Mistress".
It didn't help that by the time I had caught up to reading his currently-published stuff like "Time Enough for Love", "The Number of the Beast", and "To Sail Beyond the Sunset", he reportedly had a blood-flow problem to half his brain which made his writing a bit... "odd" is probably a kind term.
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