Witches Abroad (#12, Discworld)
Part of the series Discworld
Once upon a time there was a fairy godmother named Desiderata who had a good heart, a wise head, and poor planning skills—which, unfortunately, left the Princess Emberella in the care of her other (not quite so good and wise) godmother when death came for Desiderata. So now it's up to Magrat Garlick, Granny Weatherwax, and Nanny Ogg to hop on broomsticks and make for far-distant Genua to ensure the servant girl doesn't marry the Prince.
But the road to Genua is bumpy, and along the way the trio of witches encounters the occasional vampire, werewolf, and falling house (well this is a fairy tale, after all). The trouble really begins once these reluctant foster-godmothers arrive in Genua and must outwit their power-hungry counterpart who'll stop at nothing to achieve a proper "happy ending"—even if it means destroying a kingdom.
Others in this series
Booko found 26 book editions of Witches Abroad by Pratchett, Terry
Booko collects this information from user contributions and sources on the internet - it is not a definitive list of editions. Search Booko for other editions of Witches Abroad.