Finn Family Moomintroll (Moomin Series #3) by Tove Jansson

It is spring in the valley and the Moomins are ready for adventure! Moomintroll and his friends Snufkin and Sniff find the Hobgoblin's top hat, all shiny and new and just waiting to be taken home. They soon realize that his is no ordinary hat; it can turn anything or anyoneinto something else!