This sounds pretty sinister but it has been a shocking revelation. A closer inspection of this 1852 edition (Paradise Lost) has revealed that it is bound with human skin.
George Cudmore poisoned his lover Sarah Dunn with a potion of roasted apple and milk, laced with arsenic. He was sentenced to death & was executed in front of people. His body was later sent to the Devon and Exeter Hospital for dissection where his body was stripped of its flesh and a portion of his skin fell into the hands of a Mr. W Clifford, an Exeter bookseller. That skin was used to bind Milton’s Paradise Lost & the volume will now go on a public display as a part of crime & punishment exhibition very soon & this will also feature further discussions on executions & witch trials.
By the way, Paradise Lost is considered Milton’s best work.