In truth, they aren’t that extraordinary but the simple colour palates are very attractive. They give the spines a block presence on the shelf without getting in the way of either the titles or the author name.
It is a very elegant way to stand out on the shelf.
The only thing that annoys us is the way the imprint jumps between 4th Estate and Harper Perennial.
Of course there are sound internal reasons for doing this but from a consumer point of view it is messy and confusing — destroying all the beautiful consistency of the rest of the design.