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Books bound in human flesh are a common image in both film and games, with the Necronomicon and Book of Vile Darkness both having a Tramp Stamp on the back cover.

I imagine human skin is like any other kind of skin and can be tanned and turned into leather. There’s no reason to use the face for the cover of a book, as removing the skin from that part of the body would likely be a lot more work. And the result would be lumpy and uneven. Using “leather” back and chest would be much more time efficient. But what do evil cultists know about time management?