The Sorceress of the Strand - L. T. Meade

From the moment Madame Sara arrived on the scene, she has taken London society by storm. Madame is both beautiful and mysterious, but it soon becomes clear to both Dixon Druce and his friend, police surgeon Eric Vandeleur, that there is something sinister about the woman and the goings on at her shop on the Strand. They soon become obsessed with proving her guilty of the many crimes that follow in her wake! (summary by J. M. Smallheer)