I know this is a long shot, but when I was a kid, my Nan gave me a horror book of short stories.
The only story in it I can remember is of a child who lives with his/her single mum, and has an imaginary friend. I think the friend was called Harry. There was a paragraph of the kid sitting outside in the sun and the imaginary friends shadow was on the grass (I think). In the end, the mum goes to collect the kid from school or somewhere and finds out they've already been picked up by someone called Harry, so it turned out to be a real person.
I recall Stephen King's name, though I don't know if he was just an author of another story in the book, or if he actually wrote the one I'm remembering (I'm leaning towards it not being him because I think it'd be pretty easy to find on Google if it was).
It's more I want to find the whole book, purely for nostalgia, and if I can find out what that story was called I might be able to find the book.
Does it ring any bells for anyone?