First, there's the image. You might think that comes last but, for most readers, that's the first thing they see. They see that before they read the title or author name. The image or colors catch their eye. I'm sure you've seen classic books that have their cover "look" changed over the years. The title and author are still exactly the same - but that new look can create an entirely new impression of what the book contains.
In this example, the image shows a loving couple in medieval clothing. I will talk in a later section about all the iterations it took to reach this current image, and how those changes greatly improved sales. For now, let's just describe this image here. The couple together let a reader know this is a romantic story. The medieval clothing sets the era. The high neckline and cross convey the impression of sweet / clean romance. The reader gets all of those impressions at one quick glance, which makes the target audience think, "Hey, this is the type of book I like."
Also, the couple is large enough that this is clearly understandable even at a small stamp size. It's always critical that the contrast and detail of the image perform well even at a tiny size. That is the size most potential readers will see the cover at.
Next up, the title. The title must be clear and legible. Many new authors go with exotic fonts which are nearly impossible to read at a small size. Yes, certainly, choose a font which matches your genre - and at the same time it MUST be legible at a glance. If your reader can't be sure what it says, often they keep scrolling.
The subtitle beneath the main title doesn't have to be legible at the small size. Its purpose on the cover is merely to allow its presence on the Amazon main listing, which gives you valuable key words. The same is true for the series name. Both of these must be on the cover to be part of the Amazon listing, but their purpose is more for search results rather than being visible to a browsing reader. After all, if you have too many words in giant type the cover will become a mess.
Another piece which is optional and doesn't need to be legible at the small size is the text blurb. This is either a short review that promotes the book or a blurb which lures the reader in. This will be seen at the larger size image, if the reader clicks. Its presence is more of a "mark of credibility" since it's something that many large publishers use on their book. Even if it can't be read at a smaller size, its presence on the cover conveys a sense of professionalism.
Finally, the author name should be legible. Even if you feel you aren't a well known author yet, the key here is the impression you're giving to the reader. You want THEM to get the impression that you are a respected, talented author whose books deserve to be read. You do that through the way you state your name.
So, to summarize. A large, crystal-clear image is key. If you're not using any images at all, think about why. Even a single focal image like an origami crane might suit your purpose perfectly. You need a clear, easy-to-read title and author name. You also need a subtitle and series name (if applicable) but those do not need to be legible at the smaller size.
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