Having got a child of my own the idea that you could have a son who is both unloved and is suffering from a complete retreat into an imaginary world is something that you have to rely on Gillespie's fiction to deliver.
The fact he does so and maintains a growing tension as the cuckoo boy James grows up and drives his parents to the edge of sanity and their marriage keeps you reading even when it is uncomfortable to read about the exploits of a disfunctional family.
The book focuses on the quartet of Sandra, Ken and their adopted son James and his imaginary friend David. The problem is that David only turns up after a couple of years where James has managed to already create such a gulf between himself and his mother that the idea each could perhaps love each other has already become a non starter.
You sense that as the years go by and the imaginary David makes James step up the violence and the danger this can only end badly. As we all know it's the real people and not the imaginary who take the blame.
A full review to follow shortly...