Chocky
By John Wyndham
John Wyndham's 1963 novella (it's a slight book) is about a twelve year old boy, Matthew, who has a friend he talks to: however, this conversation is only inside his head. The friend is called Chocky.
Not completely unusual in a child (the imaginary friend) but Chocky is unusual. Leaving aside the indeterminate sex (Matthew settles on "she"), Chocky asks some very strange questions, such as why are there two sexes? "She" also has some very odd views of the world. Matthew's parents become very concerned but are not sure what to do exactly. In situations like this, you can do a lot of harm trying to do the right thing.
This is a short read but a good one. The family (two parents and two children) are perfectly normal other than the fact of this strange unwanted interloper to Matthew's head. This is a long way from a story of a "demon" child or one of "possession" and it is all the better for that. Another worthwhile Wyndham read.