The Bilbao Looking Glass by Charlotte MacLeod
In ten words or less: Annoying old woman bumped off; too many suspects to list.

Review: When Sarah Kelling visits her summer home after a long absence, she finds a precious antique looking glass in the hall. It doesn't belong to her, and no one else but a trusted caretaker has a key. Her friend--soon to be fiance--art investigator Max Bittersohn--calls the police, only to be accused of stealing the piece. When Sarah and Max aattend a party at a neighbor's house, one of the hosts is killer, a spiteful gossip whose death is little mourned. Again, Max is accused of the crime. Even though Max's family is from the area, he's not one of the yacht club elite. Sarah and Max team up to exonerate him of the crime, with the help of both his and her relatives.
Why bother? MacLeod is the master of the clever, cozy mystery.