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 away to defeat Rikard, save her sister, and maybe even turn Prince Denholm back into a human being.

The Dark World is in danger and war is soon to come. On one side are Tal and the Underfolk rebels, struggling valiantly for freedom. On another side are Milla and the Icecarls, preparing to assault the Castle in order to preserve their world. And on a third side lies a most sinister force of such power that it could unleash the ultimate destruction.