Murder at the Dinner Party by C.J. Archer

The staff in high society households are expected to be unobtrusive, but a dead butler in the dining room tends to get noticed. Did he die of natural causes or was he poisoned?

Cleo is hired by the housekeeper at a Mayfair residence after the police rule the sudden death of the butler was the result of natural causes. Not convinced by the verdict, she wants Cleo to prove foul play was involved. Considering he was acting mysteriously around some of the guests, and they were equally mysterious when they saw him, murder is a possibility.
When the investigation leads Cleo to another decades-old murder and the sudden disappearance of the primary suspect, she’s even more determined to uncover the truth. But the rich and powerful don’t like airing their dirty laundry, and her efforts are thwarted at every turn. Enlisting Harry Armitage to assist her, Cleo matches wits with her suspects in an effort to discover the secrets they’re trying desperately to hide.