An Intimate Ghost by Ellen Hart

Jane gets a shock at a wedding she's hired to cater: arriving late to check in with her crew, she finds that guests are in various stages of undress on the lawn, many seem to be taking an unnatural interest in mundane objects like houseplants and the night sky, and more than a few are engaged in some very indiscreet activities in plain view. It doesn't take Jane long to figure out that someone has spiked the food at the party, most likely with hallucinogenic mushrooms. As the police arrive, the groom takes an unfor-tunate dive into an empty swimming pool and Jane realizes that if she can't figure out exactly what has happened the blame will fall squarely on her own shoulders.