Silent in the Grave by Deanna Raybourn

“Let the wicked be ashamed, and let them be silent in the grave.”

These ominous words are the last threat that Sir Edward Grey receives from his killer. Before he can show them to Nicholas Brisbane, the private inquiry agent he has retained for his protection, he collapses and dies at his London home, in the presence of his wife, Julia, and a roomful of dinner guests.
Prepared to accept that Edward’s death was due to a long-standing physical infirmity, Julia is outraged when Brisbane visits and suggests that her husband was murdered. It is a reaction she comes to regret when she discovers damning evidence for herself, and realizes the truth.
Determined to bring the murderer to justice, Julia engages the enigmatic Brisbane to help her investigate Edward’s demise. Dismissing his warnings that the investigation will be difficult, if not impossible, Julia presses forward, following a trail of clues that lead her to even more unpleasant truths, and ever closer to a killer who waits expectantly for her arrival.