It's hard to imagine now, but at one time book covers were called "dust covers," and were thrown away as soon as the owner got the book home. People liked the look of leather bound books on their shelves. Some people still do and they'll pay premium prices to have uncovered leather-bound editions of their favorite novels.
Since it was necessary to put the name of the book on the dust cover, publishers would sometimes indulge in mildly fancy lettering or woodcuts. The rule was that the dust covers had to use course paper, be cheap, and be monochrome.
What changed all that was the proliferation of posters, especially film posters. Publishers reasoned that if posters could sell films, why couldn't they sell books?
For a while book covers did what film posters did and tried to sell personalities. That was a mistake. Books and films are different media and have to be sold different ways.
Here's (above) a cover that attempts to sell "Too Much of Water" by fixing a visual image of the heroine in our minds.
Here's (above) an attempt to sell the same book by focusing on the idea of murder on the high seas at night. Surely this is the cover that really sold the book.
Publishers continued to put personalities on the covers (above) but it generally didn't work. I say "generally," because it worked for Doc Savage and Harry Potter and a handful of others.
The James Bond books (above) finally settled on a generic handsome man seen at a distance, allowing the reader to fill in his own specifics. The reader was allowed to imagine the specific character.
Large art departments arose at all the major publishers. The people who worked there cultivated an image of mystical seers, who had a mysterious sixth sense for what would sell. The front office cut them a lot of slack and authors were seldom consulted about what would appear on the cover.
It took a while to figure out what kind of covers fit modern literary novels (above). The solution, when it came, was fascinating. Since most new literary books portrayed their characters as victims and anti-heroes, the covers would portray people who were out of focus, as if viewed through tears. The lettering was jagged, as if the book were written by the trembling fingers of a traumatized sufferer.
A variant on this was the deliberately under-stated, thin line style (above) which appealed to New-Age readers. I'm guessing that the idea was to flatter the reader who perceived of himself himself as a delicate thing, a contemplater of nature and not a purveyor of what he considers evil smoke-stack industries.
Here's (above) an interesting variant on the idea that a cover should sell the mystery, not the personality of the crime solver.
Magazines gradually phased out illustration and replaced it with photography. The book cover people tried this too, but to no avail. Buyers of fiction still preferred illustration. I'm glad they did, but I wonder why.
Of course, there was the occasional photo cover (above), even on paperbacks. Some of them sure pop out!
Photos currently dominate non-fiction covers. The cover designers still attempt to appeal to the unconscious yearnings of their readers. Here the reader is flattered by the association of reading with high culture and timeless architecture. The print is low-key and seems to say, "We readers may be quiet and unassuming, but we make the world work, so how about some respect!?"
Finding the right cover for a literary novel like "Catcher in the Rye" can be tough. Here's (above) a cover, maybe by Fletcher Stone Martin, in the scratchy sensitive style.
The first paperbacks attempted lurid realism (above), a style I usually like, but in this case it didn't fit the subject.
Finally the book was successfully issued in plain red, the cover of Caulfield's hat. The publisher threw in the towel when he realized that he'd never find a picture that captured the flavor of the novel.
I'll close with a couple of examples of the eccentric but always interesting cover style of Victor Gollancz.
Gollancz was a radical left publisher (he published Orwell) who defied the common wisdom by making chatty covers without pictures. Most were yellow because that color read best in railway terminals. Gollancz must have worked closely with his authors because all the books of his company that I've seen have a common author's ethos, that of a friendly, creative, and passionate man who's eager to engage in argument about subjects that most people never think about.
I have a few of his books. One is a book that gives star ratings to the great classics of English literature. What an odd but wonderful thing to do! Another claims that the 17th century was the greatest of centuries, and yet another is a defense of fascism, which attracted a lot of socialists in its early days. I don't have the slightest sympathy for either fascism or socialism, but Gollancz is high on my list of people I wish I could have had dinner with.
BTW: In my opinion the author shouldn't be permitted to chose the cover unless he has exceptionally good taste and market savvy. Most authors will chose a picture that conveys what the book was trying to say, and this is sometimes a mistake. The art department, if it's a good one, will add to what the book was saying. They're concerned with selling a lifestyle, something most authors don't care about, but which has tremendous reader appeal.