The Ghost of Andrew Neville

When Jacob and his parents move to a mansion in the countryside, Jacob is not surprised to find so many ghosts. Of course there would be ghosts in such a big, old place!
But why are they trapped on Earth, and what is their reason for haunting? And who are the mysterious twins that appear on the same date every year? Jacob doesn't mind sharing his room with Andrew, young master of Neville Mansion; even though Andrew died a hundred years ago, he still has a lively sense of humour.
Jacob's parents can't see the ghosts, so it's up to Jacob to prevent the third-floor ghost frightening his mother when she wants to redecorate, and help the old lady with white hair to find her lost necklace. But what is the connection between Andrew's death and the restless ghosts?