A. E. W. Mason 
The Affair at the Semiramis Hotel [EPUB ebook] 

BOOK 2 IN THE INSPECTOR HANAUD SERIES, in which we again join Ricardo and Hanaud, this time in an ambiguous situation. A young, wealthy vagabond English man, Calladine, whom Ricardo knew before, hastily comes to Ricardo’s London home in the morning, while Hanaud happens to be visiting. Calladine, very agitated, still dressed formally as for an evening ball, tells his disturbing story– He had gone to a costume party that night in a hotel ballroom, met a beautiful young woman, Joan Carew, with whom he danced, dined, and talked.

‘She was young, fair, rather tall, slim, and very pretty; her hair was drawn back from her face with a ribbon, and rippled down her shoulders in heavy curls; and she was dressed in a satin coat and knee-breeches of pale green and gold, with a white waistcoat and silk stockings and scarlet heels to her satin shoes. She was as straight-limbed as a boy, and exquisite like a figure in Dresden china.’

While dancing, Joan’s eyes fixed upon a stout costumed lady, obviously wealthy. Calladine was puzzled, but didn’t ask why the fascination with that woman. Joan and Calladine leave the party and Joan runs off at 1:30am. A few hours later, Joan appears at Calladine’s apartment door, very distressed. He lets her in. She tells her story about being obsessed with the pearls the lady wore; about finding the lady’s dropped key to her hotel suite; about sneaking into the room at night to steal the pearls but being surprised by other thieves already in the dark room, who grabbed her and tied her as she fell unconscious; about awakening in the early morning and finding herself untied, on the couch, with the wealthy lady motionless in bed, apparently dead; then fleeing, seeking Calladine’s help.

Ricardo and Hanaud accompany Calladine to his home. While Calladine showers, Hanaud finds hidden beans of mescal, a hallucinogenic plant. On the mantle, there is a porcelain figurine of a woman looking exactly as Calladine described Joan. Hanaud’s astute mind wonders, was Calladine’s story truth or a fantasy imagined in a vivid mescaline dream? He and Ricardo then endeavor to solve this mystery.