A book cover used to be encountered at full size, face out on a table, close enough to touch. Now the first meeting is almost always a thumbnail in a store listing: roughly sixty pixels wide, in a grid of forty competitors, being scrolled past.
That one fact should drive most of the decisions. A cover that only resolves at print size is optimised for the second impression, not the first.
The title is the image
At thumbnail size there is no room for a scene. Faces become dots, landscapes become texture, and fine illustration becomes noise.
What survives is the type. This is why so many covers that work well online are essentially typographic: the title is the visual, set large, with the letterforms themselves doing the emotional work. Treating the type as the illustration rather than as a label on top of one is usually the difference between a cover that reads and one that blurs.
Contrast beats detail
Detail is the first casualty of scale. Contrast is the last.
A cover with two clear tonal zones will still communicate at sixty pixels, even when every fine element inside it has dissolved. A cover with beautiful mid-tone rendering and low contrast turns into a single grey rectangle. When I am choosing between two directions, I squint at both at thumbnail size, and the one that still has a recognisable shape wins, almost regardless of which is nicer at full size.
Test at the size it will actually be sold at
This sounds obvious and is skipped constantly, because designers work zoomed in.
The practical version: keep a small artboard beside the main one, at real listing size, and look at it far more often than feels necessary. Better still, mock it into an actual grid of other covers in the same genre. A cover that looks confident alone can look timid the moment it is surrounded, and you only find that out by putting it there.
The spine and back are still real
Digital-first does not mean digital-only. The physical copy is the one that ends up on a shelf in someone's home, spine out, for years.
A spine is a brutal constraint: a few millimetres of width, with the title, the author, and usually a publisher mark all competing. It is also the version a reader sees most often after purchase. Designing it as an afterthought once the front is approved is how you end up with a spine that is unreadable at arm's length.
Genre is a language, not a cage
Readers browsing a category are reading visual signals long before they read a word: the typeface, the colour, the amount of space. Those conventions exist because they work, and ignoring them entirely usually means the book gets sorted into the wrong mental shelf.
The useful move is not to reject the convention or to copy it, but to meet it and then break one thing deliberately. The recognisable part gets the book shortlisted; the broken part is what makes someone stop.
The cover has one job
It is not to summarise the book, and it is not to be beautiful in isolation. It is to make the right reader stop scrolling and click, and then to still feel right when the physical copy arrives.
Those two jobs pull in different directions, and holding both at once is most of the work. You can see how that resolved on Building Bridges, a cover about connection in a polarised world, where the challenge was carrying real warmth at a size where warmth is the first thing to disappear.