One element of good book design, or page layout for any print or digital application, is the grid. The grid could be compared to the rough framing of a building.

One element of good book design, or page layout for any print or digital application, is the grid. The grid could be compared to the rough framing of a building.
How does one create a book cover? Does it materialize in the mind of the designer in a brilliant moment of inspiration? Or is it a formula in which the designer plugs in the genre of book and out pops a cover?