1. In the third installment a new backstory has to come up that changes the way you interpret the main story.
2. Anyone can be killed even main characters people love.
3. Relationships need to be redefined.
4. What you promised in the first installment has to be delivered in the third - but not in the way you expect.
5. People need to feel a sense of resolution and that it was worth following these characters along for the ride.
All of that happens in interesting ways. And while I knew from the word go that this structure would be adhered to he still delivers pleasant surprises in the way he executes them.
This is probably why Fix doesn't experiment with structure nearly as playfully as The Flux, the second book did. He needed to world build nearly from scratch and that demands narrative discipline. (Something Steve King has never fucking figured out and why his endings often suck.)
It's wonderful the way he makes you look at the main characters in different ways yet still remain consistent with who they are and why we liked them in the first place. A lesser writer (I'm looking at your Patricia Cornwell) would have left you hating at least one if not two of the main characters. He makes you love them despite their flaws and the damage they have created.
All of this leads to the massive showdown in Europe that has been the Checkov's Gun of this series. He takes you there with the narrative logic and instinct of Golden Era Norman Mailer.
I only have one slight quibble with the book (and any review should contain something to bitch about.) There is a scene with teenagers exploring their sexuality During that scene he describes their experience in very adult (not adult in a dirty way) terms. I quite frankly don't believe that two 13 year olds would be that mature about their desires and be able to navigate highly intellectual issues of consent using the type of language they do. They would be acting instead on exploration and instinct. But that's two pages of a long novel so not a big deal.
Overall a spectacular end to an interesting trilogy. If Wes Craven can find a copy in the afterlife he'd approve.