She Faded Into Air

"Adept at laying one icy finger on the back of your neck" - Spectator
Evelyn Cross enters a London apartment through the only door. Then vanishes. Now private detective Alan Foam is on the case.
In order to get her fortunes read, Evelyn Cross enters an old mansion in Mayfair along with her father to visit Madame Goya. Tired of waiting for what seems an inordinate length of time, her father enters Goya's room, only to find no trace of his daughter. Goya says she has already left. Yet how did she leave with only one door? Various witnesses confirm Evelyn hasn't made an exit. Even after builders are brought in remove panels and floorboards, no secret hideaways or exits are revealed.
Where is Evelyn? How has she faded into thin air? This is an impossible case for the cynical private detective, Alan Foam and he must solve it.