The covers are only part of the book, earlier work by the cover expert are also featured in the book, such as interior design, jacket sketches or art.
In my opinion, there are only few fields of design that require as much synthesis ability as cover design. On a small surface, you need to express the content of a whole book, and make the whole thing attractive. Mendelsund’s covers always are spot on, that’s what makes him one of the greatest cover designers around, not just aesthetics.
To get a previes of the book, you can see some of the designer’s creative book covers in this post.