This is a view of book covers on a standard Samsung S5 smartphone. I've laid a postage stamp on top of the phone for size comparison. That's right - your book cover is going to be seen at a size SMALLER THAN A POSTAGE STAMP when the user makes a decision to click or not click. And your cover isn't in isolation. There will be a variety of other covers listed on the same screen, usually in search results or browse results. If your cover doesn't stand out, the user will simply keep scrolling.
Another key thing to notice here is that the images are shown on a white background. If your cover itself is white, this means your cover gets "lost" in the background. If you really want a white background, always give it a dark border around the very edge. That way your cover has edges.
Try going onto your smartphone or tablet (or borrow a friend's) and search for "Hamlet". Look at the variety of covers that come up. Which can you read? Which look like a blur? Which seem appealing? Which would you scroll past?
The more you examine covers on a smartphone-sized device, even if it means using a PC and standing a distance back so the images become small, the more you get a sense of how to design your cover.
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