Birds of a Feather by Jacqueline Winspear

An eventful year has passed for Maisie Dobbs. Since starting a one-woman private investigation agency in 1929 London, she now has a professional office in Fitzroy Square and an assistant, the happy-go-lucky Billy Beale. She has proven herself as a psychologist and investigator, and has even won over Detective Inspector Stratton of Scotland Yard’s Murder Squad—an admirable achievement for a woman who worked her way from servant to scholar to sleuth, and who also served as a battlefield nurse in the Great War.
It’s now the early Spring of 1930. Stratton is investigating a murder case in Coulsden, while Maisie has been summoned to Dulwich to find a runaway heiress. The woman is the daughter of Joseph Waite, a wealthy self-made man who has lavished her with privilege but kept her in a gilded cage. His domineering ways have driven her off before, and now she’s bolted again.
Waite’s instructions are to find his daughter and bring her home. When Maisie looks into the disappearance she finds a chilling link to Stratton’s murder case, and to the terrible legacy of The Great War..