Piilomaan pikku aasi

This is one of Finnish children's literature classics, and should really be translated to many languages, because it is a wonderful little fairytale about the Duke and Duchess of the “Hidden country” where donkeys, dogs and cats live; how the country was taken over by the evil mule magician and his fox minions, and how the Little Donkey (pikku aasi) saves everyone, with his friends.
It's full of magic and adventures, and if you like OZ books or Uspenski, you will love this one, too.