To be honest, I don’ t have THE answer to this question. It’s important a book cover is clear in its message, easy to read, and makes a connection to the story. HOW they do this is totally up to you. It could be through unique fonts, awesome images, or a brand logo. It could be simple or abstract.
Regardless of how you do it, your cover should make a reader WANT to know more about the book. It’s their first glimpse into what lies inside. The cover grabs their eye, makes them pick it up, and read the synopsis. If your book doesn’t look professionally designed and doesn’t capture the reader’s interest, then it might be a “good” cover, but hasn’t fully met the definition of a “great” book cover.
What grabs your eye as a reader? What do you look for when you pick up a book?