The ghost behind the wall - Melvin Burgess

Twelve-year-old David lives with his dad in a block of flats and is a bit of a brute. He's very lonely as is old Mr Alverston, another Mahogany Villas resident, who is afraid that he is losing his mind. David discovers that he can get into the ventilation shaft of the building and begins to spy on the other residents. One day he comes face to face with a ghost, at first friendly but more and more terrifying as it becomes clear that he has a particular grudge against Mr Alverston. Soon the old man and David are in great danger and, in a frightening and utterly moving conclusion, Mr Alverston and the ghost of the boy he once was, reunite, allowing the old man to die peacefully.