The Masked Lady and The Murder — Lindsay Downs

When Lord Robert Markson, Viscount of Hampshire, is force to return to England to find out who murdered his father and older brother he's in for a surprise.
It comes in the guise of Lady Kristina Rosewood, daughter to the Earl and Countess of Crossington. To many she's quartz but to him a multifaceted diamond.
While recovering from an attempt on his life which thrust them closer together they work through emotions for each other while sorting through letters sent to his mother years ago. Slowly, they're able to discover the one man who could have set the murders in motion. The only problem, he's been dead for years so who could it be and more importantly why now.
Once all the evidence is compiled the answer is something neither could believe as the threat comes from within the late viscount's house.