The Slaves of Paris. by Émile Gaboriau

In this, Gaboriau's penultimate Lecoq novel, Lecoq doesn't make an appearance until the last few chapters of the book. In fact, the protagonists' identity remains unclear until almost halfway through. They're not missed, though, because the antagonists are a group of blackmailers of exhaustive ingenuity and knowledge, and piecing together the game they're playing with several noblemen and women occupies all of one's faculties for most of the book.
Young love, old love, forbidden love, lost love, along with a couple of missing individuals : what is the blackmailers' endgame ? Will Lecoq be able to figure it out in time ? Called "French sensational" in its day, Lecoq's last case is still sensational today.