In the final book we not only learn who has been behind all the killings but also learn of Mayhem and Graces past and what caused not only their painful split and her to turn away from profiling but also about his and Bryar's past. It feels like two steps forward and one step back even though they are getting closer to the killer and not only do we have the tensions of the little details coming together but also the tension and emotions surrounding Mayhem and Grace as those two are as stubborn as one another!
This was the perfect ending to the series as not only is the killer caught, but also we get to see where everyone in SO team one are and it also has an opening to another!