Blindfold — Patricia Wentworth

An elegant English townhouse conceals a viper's nest of greed and evil in this riveting tale of romantic suspense from the author of the Miss Silver Mysteries
 
Flossie Palmer is in the drawing room of No. 16 Varley Street pretending to be someone else when she gets the shock of her life. In the six-foot, gilt-framed mirror against the wall, a black gaping hole appears where there should be glass. A man's bloody head comes into view, followed by a hand trying to claw its way out of the darkness, and then another face with cruel, staring eyes. Terrified, Flossie flees for her life.
 
Newly returned from Paris, Miles Clayton has come back to London on a mission. His employer, a wealthy American, wants Miles to find his long-lost niece so he can bequeath her his enormous fortune. All Miles knows is her name: Miss Macintyre. When Miles and Flossie meet by chance, he has no idea that she could be the woman he's searching for. And now someone has attempted to kill the housemaid Flossie was impersonating -- but who was the intended victim?
 
As Miles moves closer to the truth, he uncovers a tangled family history of lies and lethal secrets.