The Chocolate Monster by Jan Page
Lucy Locket lost her pocket,' teases Lucy's brother, Matthew.
Lucy doesn't lose things, she insists to her brother, they just disappear! But who'd have guessed that a monster under her bed was taking them? Finally Lucy discovers the monster and his secret passion - chocolate. Before too long, she enters an excellent bargain with the greedy little monster. He gets all the chockie he wants, she gets all her things back...but Matthew's stuff suddenly starts vanishing!