This book was hugely disappointing. There's a satisfying end to the series (at least, somewhat satisfying), but 80% of this could have been removed and the effect would have been the same.
I have so many problems with this book. Huge spoiler cut ahead!
(view spoiler)[For a start, there are far too many named characters. Howey's strength isn't characterization--it's plot (most of the time, anyway). To back that up, he has characters that you can root for without getting below surface-level. The shallow characters only become a problem in Dust, which is character-, rather than plot-, driven. There are too many constantly referenced, interchangeable minor characters from Mechanical and Silo 17, and it's hard to keep them straight. And Howey gets some of the characterization wrong, too. Juliette's musings on Solo as mayor fall totally flat at the end. Is it really believable that he would ever be able to lead a group of people? (No.)
Additionally, the characters don't do anything. There are entire pages where Charlotte and Donald sit and think about the world before and the likely consequences of their actions. Juliette spends a lot of time going between levels and thinking about the children from Silo 17. It seems like they outlived their freshness as protagonists, and given that, it would have been more interesting to read about a new character from Silo 40 that had peripheral interactions with them. And then Howey introduces new characters just to kill them off - like Darcy. I think that death is supposed to be Meaningful or Tragic, but the pages devoted to Charlotte and Darcy together are really just wasted space.
It would have been MUCH cooler if the entire action of this book had been condensed into the first 20%, with the last 80% covering the unsolved mysteries. When Juliette and co. get out of the death fields (lol what was that? just green grass right next to the argon field?), they have no way of knowing that Silo 1 is gone--but then Charlotte shows up. At that point, they could have tried to reach out to any of the survivors in the other silos to get them out. What happened to Silo 40? Has anyone else escaped? Are the Silo 18 people really going to leave everyone else underground?
Anyway, all of this book can be summed up with Donald's thoughts on Anna:
Again, he thought how things would’ve been different had he woken her with some trust, had heard her out. If Anna was there to help, everything would be better. It pained him, but he missed her as much as he missed Helen. Anna had saved him, had tried to save others, and Donald had misunderstood and had hated her for both actions.
YES! WAKE UP ANNA. Find out about Silo 40. Explode Silo 1. Get Silo 18 out. Contact Silo 40. There! A plot! Amazing! Or, you know, you could spend the entire book murdering the people who have the answers and then thinking deeply about how you don't know the answers. And you could leave out the totally unnecessary subplot in which Elise is married to an adult (?). Up to you. (hide spoiler)]