Don't Look in the Mirror (Vintage Paperback) by Larry Weinberg

Alice has always hated her appearance. So it's a very strange feeling when, looking in a mirror one day, she sees a more beautiful version of herself staring back. Even more surprising, the image begins speaking. Demanding to be called Alicia, the young woman in the mirror helps Alice get a sophisticated makeover. And a cute guy at school begains to take notice of her. But what does Alicia want in return? It isn't very pretty and Alice is soon fighting for her very life.