Practical cover design resources
Indie authors are universally advised to hire someone for cover design instead of doing it themselves. It's as if everything about self publiching can be learned, but not cover design: book design, writing, editing, formatting, marketing, web design, SEO. But not cover design. Even rocket science looks less intimidating and arcane.
Assume someone dares to think of learning something about cover design. Not for actually designing covers, mind you, but mere intellectual curiosity. Are there useful practical resources on book design? Any ebooks, sites or blogs?
I am primarily interested in resources on design, layout and composition rather than image and page editing. By practical I mean that they should provide examples and tips, or be directly applicable for actually producing covers.