Perhaps the best illustrator of Science Fiction book covers from the 1950s and 60s was Richard Powers. He combined abstract imagery with SF themes to produce stunning covers for some of the best SF books ever published.
I have had an alert for the sale of one of his prints or paintings and one finally appeared on eBay. Unfortunately the starting bid at $650 is beyond my means. If I had the money, though, I would bid on this. It is an abstract view of Port Antonio.