Carolyn Wells 
In the Onyx Lobby 

A respectable young attorney in New York City, Otis Landon has barely settled into his new living quarters when an incident occurs in a neighboring apartment that he cannot, in good conscience, ignore. Robert Pembroke, a vicious, miserly man, has been murdered behind locked doors. The only people who had access to the victim were his servant and his niece. The latter, Miss Janet Pembroke, seems the suspect most likely to have eliminated her uncle with a hatpin, but her obvious distress and gentle demeanor convince Landon she is innocent. Besides, he may be falling in love with her. Obsessed with proving Miss Pembroke’s innocence, Landon follows a perplexing chain of evidence that includes a railroad schedule, a key to a safe deposit box, ticket stubs to a music hall performance, and a monogrammed handkerchief. But with time running out and no solution in sight, he must turn to Fleming Stone, the only detective smart enough to make sense of it all. (goodreads)