There's nothing quite as evocative as vintage SF artwork. There is something about a depiction of the future which is so firmly styled in the past which is very pleasing. Ace Doubles are a particular joy although, because Ace Books were so completely cavalier about record keeping, rights, credits and so on it is normally impossible to give proper credit to artists.
The first book below is one I picked up recently, not just for it's joyous cover art but because it's a pretty much perfect combination of authors (Ace Doubles were two novellas printed back to back and upside down, each with it's own cover). Vulcan's Hammer by Philip K Dick and The Skynappers by John Brunner, Ace Double D-457, 1960.
The second book, I'm particularly proud of since is contains the signature of the High Priestess of fantasy and SF herself, Ms U K LeGuin. Her classic work Planet of Exile is partnered rather oddly with one of Thomas M Disch's least sucessful books Mankind Under the Leash which, if it helps give an idea, was also published under the variant tiltle The Puppies of Terra. The two are together in Ace Double G-597, 1966
The third book below was simply one that I picked up because it's a rule... never see a vintage Ace paperback in a secondhand bookshop and leave without it! Two tales I've not read by A Bertram Chandler from 1965 (Ace Double M-129): Empress of Outer Space and The Alternate Martians. The artwork on this book is credited to Jerome Podwil with interior art (consisting of only two b/w vingettes at the head of each novella) by Jack Gaughan
Finally, the fourth book, not an Ace Double or even single but I pulled it off the shelf at the same time beacause it seemed to me that it's got to win some kind of award for dynamism in cover art: The Genocides by Thomas M Disch (signed as well as it happens!) published in 1965