Silence of Deceit by Cara Devlin

Secrets and lies simmer to the surface in the third riveting Bow Street Duchess mystery.
For years, the Duchess of Fournier has buried a shameful secret connected to her remarkable, if bizarre, ability. While Audrey’s gift has helped her solve cases alongside the captivating Bow Street officer Hugh Marsden, she longs to forget her previous committal to an insane asylum. But now, a blackmailer with a taste for violence is threatening former patients of Shadewell Sanatorium, and it appears the ghosts of Audrey’s past have finally found her.
Since their last inquiry together, distance and distraction have helped Hugh restrain his forbidden attraction to the untouchable Audrey Sinclair. However, when a woman’s body clothed in one of Audrey’s gowns, and bearing her calling cards, is fished out of the Thames, staying away from the stubborn duchess proves impossible.
Hugh and Audrey must join forces to unravel a puzzling revenge plot, but after the scrappy street boy Hugh has taken under his wing is attacked and left near death, they begin to realize just how steep the cost could be for crossing paths with a killer.