Princess Mirror-Belle by Julia Donaldson
Ellen gets a big shock when her double appears out of the bathroom mirror. Only Mirror-Belle is a double with a difference! She is a princess, and a very mischievous one at that. Whether she's wreaking havoc in a department store, riding the ghost train at the fair or seeing off bullies in the playground, Princess Mirror-Belle is a princess with a purpose - to cause as much trouble as possible!