Another plot twist I will never be over is the resolution of Jilliane Hoffman's "Last Witness". Because... The killer? Is a character that's been at the fringes of the investigation for not only one, but two books. Someone who's been part of the ensemble cast of task force cops, court officers and judges, secretarys, lawyers, attorneys, and other criminal investigation-related people. He's been around the entire time from day one, but you don't pay much attention to him because of course there have to be colleagues of the main characters, and most of them are barely more than set dressing.
And then you get one hint that the killer is someone in that whole system and then, ten pages and some frantic searching later, comes the moment when all the little hints come together. All those little hints that you didn't think about twice, because, well, this is a crime novel, someone has to supply some info and sometimes someone has to turn a blind eye for the main characters to get away with their not-entirely-legal behavior, but not in a million years you would have expected that person to be the orchestrator of it all.