(and I think it works for lots of products)
Is the purpose of the cover to sell books, to accurately describe what's in the book, or to tee up the reader so the book has maximum impact?
It's the third because if the book has maximum impact, then word of mouth is created, and word of mouth is what sells your product, not the cover.
Tactically, the cover sells the back cover, the back cover sells the flap and by then you've sold the book. If those steps end up selling a book that the purchaser doesn't like, game over. So you have to be consistent all the way through and end up creating a conversation after the purchase. Books are better at creating conversations than most products (when was the last time you talked about a pool cue), but there's lots of opportunity here, no matter what you make.
Some ways that a book cover can accomplish its mission:
- Iconic (because iconic items tend to signal 'important')
- Noticeable across the room (you see that lots of other people own it, thus making it likely that you'll want to know why)
- Sophisticated (because this helps reinforce that the ideas inside are worthy of your time)
- Original (why bother reading a book you already know)
- Generic (reminding you of a genre or another book you liked, not generic as in boring)
I don't know about you, but I judge books by their cover every day.