Looking through the books section of archive.org for scanned volumes on bookbinding and cover design (what, that's not how all the cool kids spend their Friday evenings?), I came across Modern Bookbindings, Their Design and Decoration by Sarah Prideux, published in 1906. It features the work of people and institutions who rebound books published by others, so each volume featured is in effect a one-off work of art. Though the illustrations are sepia photographs, there are some eye-catching examples of design in there. Click for much bigger views.
On another note, I may well be the last person to find out about the site Awful Library Books, but if you haven't yet been there, and want to learn about delights like Creative Recreation for the Mentally Retarded, then this is a site for you!