Third and final book!
This one spins outward to encompass a wide range of characters, shifting POV and timeframe and place. We see old events from new eyes and new eyes on old characters, and we even spiral back all the way back to before it even began.
There’s a number of angry reviews of this one out there, complaining it didn’t explain itself in the end. I find this curious because there are a great many answers here – some are more implied than others, but they fit together pretty easily and it’s explicit about the central answer of what caused the transformation. If anything, it had far more answers than I needed, though it handled the revelations quite well.
This one, I think, fully departs horror despite containing a lot of it. There are enjoyably horrible events within it – a watching sky, the depiction of the epicenter, the uselessness of containment, self-murder, the purpose and lack of purpose behind it all – but by and large it’s about different people trying to make sense of or simply make peace with what’s happening, and as whole you find out quite a lot about how things work without it searing your brain. Even the characters are all moving forward for answers, and the reader at a further step removed has no fear of what they’ll discover. This is a bleak setting, but not, as handled, a terrifying one. (Quite mineable for creepy ideas to throw at people in new contexts, though!)