I've always liked the dynamic covers that Ron Turner used to do back in the day.
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His art took the 1930s pulp covers , turned down the sex (he was working for UK publishers in the 1950s here) and turned up everything else to eleven - or sometimes twelve. I only have a few books with his covers and they are all pretty terrible. But who wouldn't want to read something written by someone called 'Volsted Gridban'? (A pseudonym used by John Russell Fearn and E C Tubb).