Oh, I'm so happy I found this thread! I've really become fascinated with book cover design, as well as bookmaking, lately... For those interested in cover design, might I recommend checking out these books...
Fully Booked - "a collection of current cover art and book design that strikes a crucial balance between sophisticated visual and content design on the one hand, and the market’s demand for availability, legibility and durability on the other. In addition, the book presents a choice selection of artist books that push the boundaries of conventional book design. Fully Booked also reveals developments in today’s graphic design and is an inspirational resource for creators and book lovers."
(If you're interested in buying it, I recommend buying it from Amazon, not Gestalten, because it's
much cheaper...I gave the Gestalten link though because you can see some previews of the book there.
)
Also, graphic designer Chip Kidd climb to fame was mainly due to his work on book covers, he has a book,
Chip Kidd: Book One of all his design from 1986-2006, and it is absolutely stuffed with book cover designs...
If you have interest in design aspect of handmade books and covers, I would also recommend:
The Penland Book of Handmade Books: Master Classes in Bookmaking Techniques.