Roughly WWII to the end of the 1960s marked the heyday of the era of Pulp Magazines and Paperbacks. The most significant improvement was the publication of Pocketbook's The Good Earth in 1938, the first paperback with cover art. The art was designed to be an attractive, eye catching lure to the novel within. It wasn't long before publishers discovered that the hardboiled school of writing was the most popular genre. Taking a cue from pulp magazines Black Mask, Argosy, Amazing Stories, Dime Detective, Horror Stories, Spicy Detective, etc., etc., publishers soon discovered that the more dark, salacious, sleazy and suggestive the cover the bigger the sales.
In the 1950s and 60s paperbacks expanded their market into Beat (beatnik), JD (juvenile delinquent), Drug, Slut, and Hard & Softcore Porn (including all its deviations).
Paperback Cover Art
The decline came when the publishers towards the end of the 60s either didn't want to pay cover artists anylonger or tried for a more modern look to compete with popular men's and women's magazines using photographic images. These new covers were stripped for the most part (by comparatively unimaginative photographers) of the dark atmospherics associated in the trashy, lurid, suggestive images of old.