This question is so easy to answer – any book in the Discworld novels series by Terry Pratchett. If I had to pick just one then it would be:
This is by far the funniest Discworld Novel in my opinion. Basically it tells the story of the three original witches- Granny Weatherwax, Nanny Ogg and Magrat – as they travel through various fairytale-enchanted places while hunting down Granny Weatherwax’s estranged sister Lilith. Along the way Pratchett parodies the Lord of the Rings in the funniest river scene ever, Sleeping Beauty and Cinderella.