I agree on Ray Bradbury's books (a bit more artsy then Asimon [did I recomend him to you in this thread already?] or Clarke - but still excelent).
Then there are many other books that would be simply excelent (I'll just go by the authors):
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Karl May (I'm sure you've heard of and maybe even watched the films, but have you read them?)
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Julius Verne (I bet you read most of his work, if there's something you missed out you should go to the library now - and if you read them when you were younger then 12 you should reread them again - they give you a completely new perspective)
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Jose Saramago (his style of writing is something completely else and his books are from historycal right up to the social realism - great stuff)
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Mark Twain (remember his books are social/political satire mostly, so don't simply go for Huckelbery Finn or Tom Sawyer - although those two aren't bad either)
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Jack Karuac (he talks about things that were basicaly the inspiration for the hippie movement, so you're bound to like it
)
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John Steinbeck (what can I say - novels at their best)
As I said before: "More will follow..."