Everything you need to understand or teach Melusine by Lynne Reid Banks.
Melusine is, as its subtitle suggests, a "mystery," and each of its short chapters raises questions that keep the reader wondering what will happen next or how a bizarre occurrence will be explained. Melusine is also a bit of a fantasy in that a number of unlikely events occur. Despite this, Banks deals with several serious subjects in her book.
Essentially, Melusine is the story of a twelve- or thirteen-year-old English boy's two-week-long vacation at a mysterious decaying chateau about an hour's drive inland from the city of La Rochelle on southwestern France's Atlantic coast. Visiting with his father, mother, and twin younger sisters, the boy, Roger, becomes increasingly curious about Melusine, the strange and sometimes secretive daughter of the estate's impoverished owner, Monsieur Serpe. As he and Melusine become friends, Roger finds more and more to be curious about. Why is she so afraid of her father...