Frogkisser!

Author: Garth Nix

Princess Anya has a big problem: Duke Rikard, her step-stepfather is an evil wizard who wants to rule the kingdom and has a habit of changing people into frogs, and her older sister Morven, the heir, is a wimp--so with the help of the librarian Gotfried (who turns into an owl when he is upset), and the Royal Dogs, she must find a way to defeat Rikard, save her sister, and maybe even turn Prince Denholm back into a human being.