I'm sure that there was an entry for that Harper Collins design competition they did a few years back, whereby the spines not only had a picture on them (it was like a stylised representation of a landscape) but the books could be placed in either Chronological or Publication order and the spines would still form a complete picture in either case.
Can't seem to find it anymore, but i thought it was a really fun idea.
EDIT: Here they are:
That's a really cool design and I love that the books can be displayed in both orders! It's a shame that they never got produced as I feel they would've been quite popular (well among us at least)
'It is not easy to throw off in half an hour an enchantment which has made one a slave for ten years' - The Silver Chair
EDIT: Here they are:
Those really are excellent! I wish they'd been published for real — I would love to see more of the designs for the covers, too, as well as for the spines. That's a style that manages to look timeless — both classic and modern at the same time (the mountains on the spines remind me a bit of some of Tolkien's artwork!) — and it's far more striking than the currently popular generic-fantasy-art that's on the covers of the upcoming new editions. I wish HarperCollins would catch on with something like this, but clearly they're not likely to.
"Now you are a lioness," said Aslan. "And now all Narnia will be renewed."
(Prince Caspian)