I think this series is music book. because these storys are each have one of popular melody.
On Halloween Leo and his friends are prepared halloween party. They get maskin front of big doors. At that moment they see a ghost. but the ghost is gone. They dress up halloween clothse and masks. Then they get in the party place. It is Buckingham Palace. but there are ghost too. Leo think ghost talk to we something. They fallow the ghost. There is very wonderful place.