In a dark house / Deborah Crombie.

Enhanced descriptions from Syndetics:

Scotland Yard detectives Duncan Kincaid and Gemma James return in a chilling and suspenseful tale of murder, kidnapping, madness, and the frightening ordeal of a helpless child, the latest masterwork of crime fiction from New York Times Notable Book of the Year author Deborah Crombie ...

An abandoned warehouse burns next door to a women's shelter for victims of spousal abuse, an apparent case of arson. But it is the charred corpse within -- a female body burned beyond all recognition -- combined with the political sensitivity of the case, that entangles Superintendent Duncan Kincaid in its twisted skein.

At the same time, Kincaid's lover and former partner, Gemma James, is coping with twin crises of her own, one personal and the other professional. Gemma must put her private concerns aside to investigate the disappearance of a hospital administrator, a beautiful, emotionally fragile young woman who vanished without a trace. Yet neither Gemma nor Kincaid realizes how closely their cases are connected -- or how important the resolutions will be for a young child who was a victim of parental abduction.

In an old, dark, rambling house, nine-year-old Harriet worries about her feuding mum and dad, her friends, her schoolwork. Most of all, she worries about the strange woman who is her only companion in this scary, unfamiliar place. The events that led her there happened too quickly and are too complicated for a child to fully comprehend. But despite her youth and innocence, Harriet's awful fears will not be silenced: that she may never see her parents again ... and that her own life is in serious peril.