Dorothy Must Die by Danielle Paige

A sequel of sorts to The Wizard of Oz, Dorothy Must Die follows Amy Gumm, a high school student whose life suddenly changes when she's swept up into a tornado and dropped into the magical world of Oz. This Oz isn't the same one everyone remembers, though. It's been taken over by Dorothy Gale, the former protagonist, who has somehow become a deranged dictator, dead-set on sucking up all of Oz's magic for herself. Now, it's up to Amy, and a group of formerly wicked witches to take Oz back and defeat Dorothy once and for all.