Fantasy novel, the sequel to The Weirdstone of Brisingamen, by Alan Garner, British children's writer.
Less successful than Weirdstone, this is still an excellent book with some exciting and scary stuff in it. Colin and Susan are, once more, the central characters. Susan becomes possessed by a rather creepy entity called the Brollachan which enters and manipulates its hosts. Susan is restrained, and Colin goes on a moonlit run across the fell tops, an eerie path marking his route, to find a flower which will expell the Brollachan from his sister.
The story rattles on with a kidnapping - Susan gets locked up in a house which only exists when the moon shines on a ruin. They light a fire on a burial mound and reawaken a horde of ancient soldiers. Susan blows a horn which releases the magic of the earth, but which will forever ring in her ears.
Excellent stuff. Like books used to be.