This is one of those books where I can understand why you didn't like it. The mystery isn't all that mysterious. Margot is almost dislikable (in fact she is dislikable). There are two female character of any note, and the other female characters are disliked by one and all, it seems like.
Yet, I couldn't stop reading, so it is one of those books were you know and admit the truth of the faults, but you still don't care anyway. Callaghan gets points for making a lead unlikable, and given the plot, the flaws are not all that surprising