From the delicious new hardback “The Look of the Book: Jackets, Covers, and Art at the Edges of Literature” by Peter Mendelsund and David J. Alworth:

Announcing the text and creating a conduit between imaginary and real space are two key tasks of the book cover. They are what the book cover does, first and foremost, but they are not all that it does. The cover’s job is not over when you begin reading the pages. A good book cover has that timed-release quality; it changes with you as you read.