I think that this was legitimately the first time I've read this one, and that's unusual. I don't think that there are very many Christie mysteries left that I haven't read at least once.
Overall: this was a middling mystery. Not one of her best, but far from her worst. The title integrated nicely into the mystery itself, and the solution was interesting but not surprising. As in many Christie novels, there is a strong reliance on coincidence and there are characters who are much more involved than it appears at first blush. Those Brits seem to have a positive penchant for going incognito and blackmailing unsuspecting homicidal maniacs. Why anyone would think it is a good idea to blackmail a murderer surpasses all understanding, but it seems to be a constant feature of the Christie mystery. Blackmail appears to be the national pastime, second only to cricket. And murder most foul.