This was a cover art technique that was used only rarely. The VAST majority of Gold Medal covers bore the distinctive gold spine with the medallion colophon at the base. Most Lion Books bore a solid colored spine. A few volumes, including some Bantams and Graphic Books, had covers that "wrapped," but they were actually multiple scenes depicted as murals, so the scenes varied from front to back.
I've seen some original art that depicted both covers on the same canvas (actually illustration board). But covers such as the ones above (a single image that actually wrapped around the spine) were a bit of an oddity.
The books by John D. MacDonald and Jim Thompson are relatively valuable, not because of the art, but because they are paperback originals by very collectible authors.