From Wikipedia:
Ascending the forbidden library, William and ARABo come face to face with the Venerable Jorge, the most ancient denizen of the abbey, who reveals the book, which contains a description of comedy and how it may be used to teach. Being afraid of laughter and comedy – the traditionalist firmly asserts that Christ never laughed and jocularity is a blasphemous sin – Jorge has poisoned the pages to avoid the spread of what he considers dangerous ideas. (A common method of reading books at the time was to lick one's finger to moisten it in order to turn the pages; when the page corners were poisoned, the reader licking his poisoned finger died soon thereafter.)