Hardback, paperback, leather-bound slipcase edition, fabric, vinyl, or executed priest's skin. (That's right, vinyl is for people with no taste.)
As reported in The Guardian.
A 17th century book believed to be bound in the skin of a priest executed for treason appears to bear a "spooky" image of his face on the cover, according to the auctioneers who are selling the book... The account of the death of Gunpowder Plot conspirator Father Henry Garnet is described by experts as "rare and macabre"... Sid Wilkinson, from Wilkinson's Auctioneers in Doncaster, South Yorkshire, who will be selling the book on Sunday, said he could see the Jesuit priest's face peering out from the cover.
Pretty interesting how the article thinks that the "face peering out" bit is creepy, and not, say, the dead human it's made out of. Somehow I can't see this one getting through a production meeting...