A noted detective is getting ready to go to Brazil for an important job. He drops by Notre Dame where a young woman he never met says a few sentences to him that leave him pale and canceling his trip.
A young woman, deeply in love, spurns her lover’s marriage proposal because she loves him too much.
An international celebrity is found mysteriously killed in a variation of the locked room mystery.
All these events are connected and are set in 1909 Paris, where the atmosphere is romantic and mysterious and the art of detective investigation is very much to the fore in the story. This was on a list from Michael Grost’s list for Mystery Scene magazine of classic mysteries that you should read but probably haven’t. Here is a piece about this book which I believe was written in 1907.