Bumper Fun Grimoire

This may be another name for the Monster Fun Grimoire, though The Art of Discworld shows them as two separate books.

It would appear that this is in fact a separate book: while it contains nothing overtly magical, it belongs to the one organisation which is perfectly capable of putting the "grim" into "Grimoire". The Council of the Guild of Fools and Joculators guards the book jealously, as the ultimate repository and final arbiter of all things Foolish.