The End of Oz : 4 by Danielle Paige
In this dark, action-packed fourth book in the New York Times bestselling Dorothy Must Die series, Amy Gumm travels from Oz to the twisted land of Ev, where she fights to free Oz from evil once and for all.
My name is Amy Gumm. You might remember me as the other girl from Kansas.
When a tornado swept me away to the magical land of Oz, I was given a mission: Dorothy must die. That's right, everyone's favourite Wicked-Witch-slayer had let the magic of Oz corrupt her. She turned evil.
So I killed her. But just when we thought it was safe to start rebuilding the damaged land of Oz, we were betrayed. Now I'm following the Road of Yellow Brick as it helps me escape toward the mysterious land of Ev, where the Nome King rules a bleak and angry world.
And what I'm about to find is shocking: My original mission may not have been successful. I thought my job was over, but it's only just beginning. And it's up to me to foil Dorothy's plans for revenge-and finally save the land I've come to love.