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There are two issues at play here, which I believe you are conflating. The first is the themes or "hook" which is conveyed by the imagery on the cover; the second is the professionalism of the cove...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/44898 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
There are two issues at play here, which I believe you are conflating. The first is the themes or "hook" which is conveyed by the imagery on the cover; the second is the professionalism of the cover. In my experience, I have found that both elements are necessary to earn a click. A professional cover which conveys no information, such as a plain text one, will likely not get clicked on. However, a book with an interesting concept on the cover which has clearly been created with little skill will still not likely earn many clicks. An unprofessional cover brings up many concerns about the book: How much effort might the writer have put into their actual writing, if they care so little about the cover? Is it possible that the writer is just young and unaware of conventions? If someone happens to be skilled not just in writing, but in editing, photography, graphic design, etc, they may as well create their own cover-- assuming that they can produce something of professional quality. Why create a book that is not of high quality all the way from cover to cover?