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Basically, I like the two on the left more than the two on the right, but find it quite difficult to articulate why.
However, it doesn't matter much, because in my view there's a runaway winner this year which isn't a book cover at all. This picture is copied from the creator's website, and shows the work of art as it was set up in the Exhibition Hall at Loncon 3:
Quite apart from it being an amazing and wonderful creation in its own right, it is a lovely salute to Iain M. Banks and a memory of our Worldcon. So my vote goes firmly to Tessa Farmer's Wasp Factory sculpture.
Correction: I used the wrong Mars Evacuees cover above. The correct one is this:
