Death in Five Boxes by Carter Dickson

Dr. John Sanders of the Harris Institute of Toxicology was on his way home from work at one o'clock in the morning when he was approached by a young woman, Marcia Blystone,  with an unusual request. She asked Dr. Sanders to go with her into a house near where they were standing. When Sanders asked why, she said that her father had gone in there, that she was afraid for him, and was also afraid to go in alone. Sanders agreed to go and they entered the house and started up the dark stairs toward the apartment for Felix Haye. Along the way, Sanders tripped over a sword stick which had blood stains, and met a man named Ferguson coming out of an office. Ferguson said that he had heard heard hysterical laughter coming from the apartment on the next floor up, and then everything became quiet.

When they got to Haye's apartment, Sanders went in, leaving the girl outside. He found four people seated around a table. Three of them were in a drugged sleep, and the fourth, Haye himself was dead having been stabbed. The identities of the people in the room were established. The three sleeping people were Dr. Dennis Blystone,  Marcia's father. Dr. Blystone was found to have four watches in his pockets. Another sleeper was Mrs. Bonita Sinclair who was an art critic. She was found to have  quicklime and phosphorus in her purse. The third sleeper was Mr. Bernard Schumann who was head of the Anglo-Egyptian Importing Company. He had the mechanism of an alarm clock in his pocket. The dead man was Felix Haye, a very wealthy investment broker.

Chief Inspector Humphrey Masters took charge of investigating the case. It had been established that the sleepers had been given atropine, a derivative of belladona but in varying amounts. They had drunk a cocktail which had been prepared by Mrs. Sinclair who had been watched by everyone present. She could not have put the atropine into the cocktails. The atropine was only in their glasses; there was none in the cocktail shaker. Masters and Sanders set out to investigate, and Masters decided that the assistance of Sir Henry Merrivale was needed. They found the plump HM exercising in a delightfully funny episode.

Sir Henry was contacted by the law firm of Drake, Rogers, and Drake on the day after Haye was murdered. A burglary  had occurred in their office and some things had been stolen from the items which Felix Haye had left for safekeeping there. Missing were five small boxes which the solicitors were supposed to open after Haye's death. Records there showed that the names on the boxes were Bonita Sinclair, Dennis Blystone, Bernard Schumann, Peter Ferguson, and Judith Adams. Three of these people had been present at Haye's death. Ferguson had been Haye's apartment that night, and the identity of Judith Adams was a complete mystery.

Sir Henry and Masters continued their investigation, and everyone concerned was questioned and a great deal of their history was uncovered. HM, of course, found out the identity of the murderer and explained the great puzzle of how the atropine had been put into the drinks.

Carter Dickson is the pen name of John Dickson Carr who is the master of locked room mysteries. The book was published in 1938.