An Enemy at Green Knowe
From a customer review on amazon:
This beautifully written, lyrical book is genuinely scary in a way that puts much modern children's literature well and truly in the shade. Grandmother Oldknowe is protected by all the good things of the earth--stone and water, and all the deep things of nature--in her struggle with the horrible Melanie Powers. Aided by Tolly and Ping, it's a battle for the soul with moments that will make adults draw their breath sharply. This is the fifth book in the classic Green Knowe series.
