"Can You Make Title Bigga?", by Jessica Bell, outlines not only the general principles of what makes a good book cover, in terms of what looks good and what can sell your book, but also the relationship between authors and book cover designers. There are tons of great examples of book covers in it to illustrate different principles (I thought maybe Jessica Bell was a bit hard on some of the covers compared to others, but maybe I've a less discerning eye on this).
One thing I thought was particularly useful was an appendix that comprises client questionnaires (one for fiction and one for non-fiction), where Jessica outlines methodically the kind of information a book cover designer will want to see to understand their brief (e.g. tagline for the book, what other authors have said about your work etc etc).
I think this book will appeal particularly to self-published authors who are thinking about approaching someone to professionally design a book cover for them. If this is the case for you, check this out!