House of Desire — Margaret Lucke

When a trip across time barriers leads to murder ...
At a gala fundraiser to save a grand San Francisco Victorian, reluctant psychic Claire Scanlan she encounters a mysterious young woman, Roxane, who is invisible to everyone but her. Roxane is a "soiled dove" plying her trade in the mansion in 1896. She has discovered a secret portal that lets her slip into what she calls the Future House to escape the most brutal of the men who buy her favors.
When the fundraiser's organizer is murdered in the mansion, Roxane is the sole witness. Terrified, she flees back to her own time and enlists the only true gentleman she has ever met to help her find justice for the victim.
Claire's philandering brother-in-law is accused of being the killer. To clear his name she must find the elusive Roxane-which means risking a perilous journey into the past from which she may never return.