I have just, by an utterly lovely stroke of luck (dd was lent her teacher's own copy as a special extra reading book), been able to re-read Lucy Boston's The Castle of Yew, which is otherwise out of print.
It belongs to a very special category of short, enchanting, very finely-written things - Ted Hughes' The Iron Man is the other one which springs to mind. And maybe Kipling's "How the Whale got its Throat" has the same quality.
If you know what I mean, can you add anything to the list?