Ancient and Classical Mythology
Chaffinch's Ancient and Classical Mythology does exactly what it says on the tin. It is the seminal work on the ancient mythologies of the primitive and classical peoples of the Discworld. The witch Eumenides Treason kept a copy on her bookshelf, where sinister-looking elderly volumes whose names had faded from the spines of venerable leather-bound books were visibly restrained in place by a formidable-looking steel bar and padlock. Tiffany Aching, when told to unlock the books and fetch forth the copy of Chaffinch, discovered that the shelf of perceived grimoires which had radiated evil and eldrich feelings over several generations of local villagers seeking their witch's advice, had titles like An Encyclopaedia of Soup, when viewed from close-up. Noting this as another example of applied boffo, Tiffany brought the book down for examination. Festooned with bookmarks, illustrated and randomly lettered by Don Weizen de Yoyo, Tiffany found pages Eighteen and Nineteen to be most educative concerning her dealings with the Wintersmith. On the death of Miss Treason, she was allowed to take the book and call it her own.
Annotation
The Roundworld reference is to Thomas Bullfinch who wrote The Age of Fable or Beauties of Mythology and The Golden Age of Myth and Legend amongst other books.