Murder in the Dark by Kerry Greenwood

It's Christmas, and Phryne has an invitation to the Last Best party of 1928, a four-day extravaganza being held at Werribee Manor house and grounds by the Golden Twins, Isabella and Gerald Templar. She knew them in Paris, where they caused a sensation. Phryne is in two minds about going when she starts receiving threats. She promptly decides to accept the invitation - after all, no one tells Phryne what to do. At the Manor House, she is accommodated in the Iris room, and at the party meets two polo-playing women, a Goat lady (and goat), a large number of glamorous young men and a very rude child called Tarquin. The acolytes of the golden twins are smoking hashish and dreaming, and Phryne finds that the jazz is as hot as the drinks are cold and indulges in flirtations, dancing, and mint juleps. Heaven. It all seems like good clean fun until three people are kidnapped, one of them the abominable child, and Phryne must puzzle her way through the cryptic clues of the scavenger hunt to retrieve the hostages and save the party from disaster.