The private detective is also known as a private investigator, private eye or shamus.
Police detectives are police officers who investigate crimes and pursue criminals.
Private detectives are hired by people and businesses to conduct investigations into various matters. Sometimes they too investigate crimes but more often they investigate people's backgrounds. Detectives, both police and private, are frequently the subject of mystery stories, television shows and movies. There was no doubt that the book was missing, no doubt at all. Jamie didn't know.