Roll out your book covers with a pasta machine, if you don't have one then try to use some wooden rulers on each side of your clay and roll down to an even thickness. Cover the sides and back with this sheet on your block of clay pages. At this point I would press the indentation of the book spine on the sides of the covers, don't drag a clay tool across or you will get crumblies. Make sure the page front edge is concave and the spine is convex.
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