Murder at the Villa Byzantine / R.T. Raichev.

Enhanced descriptions from Syndetics:

When a birthday party for one of their Hampstead neighbors turns deadly, Antonia Darcy and Major Hugh Payne end up investigating the murder of one of Melisande Chevret's other guests. The aging actress becomes a natural suspect, as the victim was her love rival. But after the first murder, a second takes place at the Villa Byzantine. The owner of the house is royal biographer Tancred Vane, who swears he is innocent. And surely Catherine Hope, an elderly lady helping him with his research, can have nothing to do with it. A damning piece of evidence points to the victim's daughter--but why would that a teenage girl have a dainty silk handkerchief bearing her monogram? And would she drop it so conveniently beside her mother's body? As the questions mount, Antonia Darcy and Major Payne search desperately for answers.