Sally Lockhart #3: The Tiger in the Well book by Philip Pullman

Pullman is fast becoming a modern-day Dickens for young adults. The setting is the same, the strong eye for characters is there, as are the brooding atmosphere, the social conscience, and the ability to spin plot within plot.
Sally Lockhart is now a young woman, left alone with a toddler. Nothing prepares her for the shock of receiving a summons from a man she has never even heard of, suing for divorce and the custody of her beloved Harriet. Sally struggles against the net closing around her, seeking to find out who is persecuting her and why.

The writing style is lively and direct, and there's lots of action.