The Cellar by Minette Walters

When the Songoli family moves to London, they have a secret.  In addition to their two sons, there is another member in the family who no one knows about.  Yetunde, the wife, made a trip to an orphanage before they left.  There she claimed a young girl, saying that she was the girl's aunt.  The truth is much more sinister.

Muna, the girl, has been taken in order to be the family's slave.  She is forced to clean and cook all day, her only refuge a stained, ragged mattress in the cellar with no light or heat.  She is beaten regularly by the wife and later, as they grow older, by both the boys.  She is abused sexually by the husband.  She is not allowed to have any schooling, or even to speak.

But things have a way of changing.  When the family's younger son goes missing and the police come to the house to investigate, things take a turn for the better for Muna.  She is moved upstairs into her own bedroom and claimed as the family's daughter.  They need Muna to validate their stories so slowly fall into her debt.  As the weeks go by, the family starts to realise how strong Muna really is and the lengths she will go to as she wrecks her revenge.

Minette Walter is one of the finest mystery and suspense novelists working.  Her books have layer with the story being told prosaically and then a lower level where evil things dart about and where the lies of the top layer are slowly peeled back and revealed.  Her sense of timing is immaculate and the reader is never disappointed when the story is finished.  This book is recommended for mystery readers.