The Haunting of Gaspard Feeblebunny

Young Constable Gaspard Feeblebunny is unfortunate that one evening on a lonely road he sees Death attempting to harvest the soul of Percy Pargeter which has done a runner. It is doubly unfortunate that the new whiz kid in the afterlife civil service decides that Death should have an assistant to handle such a mundane task as tracking down ghosts, because Feeblebunny is pressganged into service as a human subcontractor, though with some powers to assist with his task.
This is the beginning of a series of civil service bungles which leave the unhappy constable with a houseful of ghosts and the enmity of a psychotic Sergeant Hardcastle who is out to do him harm. Over time the ghosts are able to join their powers and take control of the young man's life. This turns out to be a mixed blessing because though they cause mayhem, they also make him a very wealthy man with a series of ventures, not all of which are a spectacular success, particularly their establishment of his flat as a ScareBnB with neither his knowledge nor consent.