Frogkisser, by Garth Nix

Princess Anya is a bit over being the responsible sister, but someone has to save the kingdom from the evil sorcerer duke Rikard. Initially setting out to find the ingredients for a lip balm that enables recovery of people transformed into frogs, Anya finds her quest expanding beyond anything she could have imagined in her cosy library in the castle. Along the way she finds help from royal dogs and robbers, wizards, dwarfs and otters and hindrance from sorcerers, witches and weasels. Can she rescue her sister, save the kingdom and transform a growing number of frogs? And will her lips ever recover from all the kissing? In a delightfully fractured fairytale Nix creates a flawed but loveable team of heroes, led by a capable and admirable princess.