Your cover will be seen at 80 pixels wide before it is ever seen at full size. Almost every design decision that matters follows from that one fact.
The thumbnail test
Take any cover, shrink it to 15% of its size, and look at it on a phone in a grid of eleven competitors. That is the actual viewing condition. Covers that pass this test share a small set of characteristics, and they are not the ones designers instinctively reach for.
What survives shrinking
High contrast between title and background. Not subtle tonal relationships — hard contrast. Cream text on pale grey looks sophisticated at A4 and disappears entirely at thumbnail.
Three to five words of title, set large. If the title needs to be small to fit, the subtitle is doing too much work. Move it down and shrink it.
One focal element. A single figure, object or shape. Detailed illustrations with six points of interest turn into visual noise.
A limited palette. Two or three colours, one of them dominant. This is also what makes a cover recognisable when a reader is trying to find it again.
Genre conventions are not clichés
Every category has visual conventions readers use to identify what a book is: the sans-serif caps and single object of business non-fiction, the moody figure-in-landscape of literary fiction, the high-contrast type and shadow of thrillers, the hand-lettered warmth of romance.
New authors often want to break these, reasoning that standing out is the goal. It is not. Being found is the goal, and a reader scanning a thriller list makes a category judgement in a fraction of a second. A cover that reads as literary fiction in a thriller grid does not look distinctive; it looks like a mistake, and the reader moves on.
Belong on the shelf. Then stand out on it. In that order.
Typography does most of the work
On the majority of commercially successful covers, the type is the design. Getting it right means a display face with genuine character rather than a system default, deliberate letter-spacing, a clear hierarchy in which the title dominates absolutely, and — critically — no more than two typefaces.
The single most common amateur signal is a title set in a font that came free with a word processor, spaced at its default, with a subtitle the same size fighting it for attention.
What quietly destroys conversion
- Stock photography used literally. If your cover shows exactly what the book describes, it is illustrating rather than selling. Readers want a mood, not a diagram.
- Too many elements. Every additional object halves the impact of the others.
- Low-resolution or over-compressed images. Visible artefacts read as "cheap" instantly, even to readers who cannot name what they are seeing.
- Ignoring the spine. If a bookshop ever stocks you, the spine is all anyone sees. It needs to be legible from two metres.
A practical brief for your designer
Give them five covers you admire in your exact sub-category and explain what you like about each one specifically — not "the colours" but "the way the title occupies the top third". Give them the emotional register in three words. Tell them the reader. And then let them design, because the entire point of hiring a professional is their judgement, not your art direction.