Damian Drooth, Supersleuth: Gruesome Ghosts by Barbara Mitchelhill
The new girl at school has long blonde hair and big blue eyes, so naturally Damian is fascinated when she says that her grandparents' house is haunted. As the local specialist detective, he feels honour bound to solve the mystery! He agrees to spend a night in their bedroom, to see the ghostly white figures for himself - and when the ghosts appear, they go into their nightly mime of one throttling the other. But this time Damian throws himself at them, and they turn out to be very solid humans wearing sheets!