Cleveland Moffett (1863-1926) was an American author, journalist and playwright. He specialised in adventure and mystery novels. His works Through the Wall (1909), Land of Mystery (1913), The Conquest of A Romance of Disaster and Victory (1916) and Possessed (1920).
I might have finished Through the Wall if I had more leisure, but after 60 pages I found the plotting a bit too convoluted and the characters not sufficiently believable.
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. A 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. It is a locked room mystery, which I normally do not like, but the way the author slowly uncovers layers truth behind the mysterious situations is already very apparent. It has the effect of a book of one cliff-hanger after another and I am hooked.
Final word: what a splendid plot and telling. Truly this is the story of a master detective pitted against a master criminal, all wound around a tale of love and friendship. I will be reading this on Forgotten Classics.
A very interesting detective, strong and often dramatic story line, well-drawn characters, interesting setting in early 20th century Paris. A certain amount of sentimentality may be jarring to some, but the events justify it.