I was originally under the impression that this was part of James' Dalgliesh series. It isn't. This is the story of Philippa. Philippa has been adopted and has always dreamed of her birth parents - her birth father in particular, as she was told that her birth mother was a maid in a Great House. She wasn't. Thanks to the (comparatively) recent law, Philippa is able to obtain her original birth certificate once she's eighteen. She's not prepared for the results. Her birth father sexually assaulted a young girl. Her birth mother subsequently murdered the girl.
This is the story of Philippa's relationship with her birth mother and her adoptive parents. It's also the story of the murdered girl's father and his desire to murder Philippa's birth mother.
This is a suspenseful, well-written mystery that is highly recommended. Do not be afraid of graphic descriptions of violence, there are none.