(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?

The third.

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 purcha