If today you were to spot a kid walking around and talking to themselves, you'd probably reasonably conclude that they were imaginative.
But if they were managing to have exactly one half of a complete back-and-forth dialogue, only without another person visibly present, then maybe they're getting a jump on wielding some unseen Bluetooth technology, right? Except that John Wynham's novel takes place the year it was published - 1968. It's an unusual and potentially alarming kind of occurrence, one that young Matthew Gore's father finds himself overhearing on a spring day just before his son's twelfth birthday.
Initially, the premise reminded me of both Ray Bradbury's "Zero Hour", and the Alfred Hitchcock Hour episode "Where the Woodbine Twineth", based on Davis Grubb's short story. Both tales concern children interacting with an ambiguous presence that either remains unseen, or cannot be seen by adults. I'm sure there are other examples. Wyndham's novel quickly distinguishes itself by making it clear that whatever is communicating with Matthew is not something outside of his person.
In much the same way that he achieved realism with his classic novel The Day of the Triffids, Wyndham again neatly combines domesticity and a grounded setting with an unmistakably SF idea. The author never goes for shock value, but instead slowly weaves a compelling and disconcerting account of a father's first-hand experience with whatever force is in contact with his son.
This is science fiction with a message, but that's a good thing, and is not achieved at the expense of excellent storytelling. Short but memorable, Chocky deserves your attention.