It is the union of beauty and truth that create a masterwork. Were a masterpiece of literature to be adorned with an ugly cover...could we still call it a masterpiece?
It seems we should, but it also seems that extreme care must be taken in manifesting a great work of art, and book covers are best when created with the highest possible intention by true artists, to complement the work within, and not done by committee for the benefit of pleasing a multitude of (sometimes questionable) aesthetics.
{At this juncture we must tip our hat to Chip Kidd, who walks with the giants--including Paul Rand--on this front: long may you reign, Mr. Kidd.}
Herewith, a great cover wraps a great work, in Weimar (again).