A cover has one job before any other: to tell the right reader, at a glance and at thumbnail size, what kind of book this is. Genres have visual habits that readers have learned to read without thinking. Following them is not copying. It is speaking a language your reader already understands. These are the common patterns.
The universal basics
Whatever the genre, a cover must be legible when small. Test it at the size of a thumbnail in a store listing. The title should be readable, the main image clear, and the overall impression recognisable even when you cannot see the detail.
Keep the design simple. One strong idea communicates faster than several competing ones.
Thrillers and mysteries
These tend to use bold, high-contrast typography, dark or moody colour, and a single focal element: a figure, a place, an object. Large author names are common for established writers. The mood is tension and unease, and empty space is often used to suggest threat.
Romance
Romance covers vary a great deal by subgenre. Contemporary titles often use bright colour, illustrated or photographic characters, and friendly type. Historical romance leans toward period styling and warm tones. The key signal is emotional tone: readers should see at once whether the book is light and funny or intense and dramatic.
Fantasy and science fiction
Fantasy covers commonly use illustration, a striking scene or symbol, and expressive lettering, sometimes with a map inside. Science fiction leans on futuristic imagery, space, technology and cleaner, more graphic type. Both reward a strong sense of a world, and series branding matters because readers return for the next book.
Nonfiction and business
Here the title and subtitle carry most of the weight. A clear promise, large readable type and a restrained design suggest authority. Avoid clutter, and make sure the subtitle says what the reader will gain.
Memoir
A memoir cover often uses a personal image, such as a photograph or an object with meaning, with a title that sounds like a human voice. The aim is intimacy and honesty rather than polish for its own sake.
Children's books and literary fiction
Children's covers use bright colour and characters the child will want to meet. Literary fiction tends to be quieter: typographic, understated, with an image that hints rather than explains.
Use the conventions on purpose
Look at the covers of the best-known recent books in your exact subgenre, and notice what they share. Then decide what you will match so readers recognise your genre, and what will make yours stand out within it.
If you are writing a series, plan the design so the books look related. To brief a designer well, read our guide on how to brief a book cover designer, or talk to Marlowe's cover design team.