This retro book cover is for Midworld by Alan Dean Foster, and dear Lord, there’s a lot going on here. You’ve got a muscle-bound marine holding some sort of bazooka in a psychedelic jungle. He’s next to a self-satisfied-looking bull and a smug-looking dragon with googly eyes. In the background are what look like pumpkins and pears. Either the pumpkins are very small or the pears are very large, because they’re approximately the same size.
Anyway, this masterpiece is by H.R. Van Dongen (August 20, 1920 – February 27, 2010), who was also sometimes credited as Richard Van Dongen or simply just Van Dongen. He frequently did covers and interior art for Analog Science Fiction and Fact and Astounding Science Fiction. He also did covers for books such as The Castaways of Tanagar by Brian Stableford and The Wooden Star by William Tenn, and he worked as a freelance illustrator for several Christian publications.
More interesting, however, Van Dongen was part of the Air Force during World War II He spent 11 months as a PoW after his plane was shot down. Perhaps his time spent on the battlefield was part of the inspiration for this book cover.
The novel itself is by Alan Dean Foster. Foster is known for his Humanx Commonwealth novels, but he also worked on the Star Trek Logs. A surprising fact about Foster is that he ghostwrote the original Star Wars novelizations (which were credited to George Lucas).
You can read more about Alan Dean Foster at his personal website.