The big bad finally made its move in the previous book. A massive structure called the Ring parked itself at the edge of the solar system and every major player is sending everything they got.
In this book we learn more about the protomolecule origins, while maintaining the mystery behind it. This is a good thing as 'in between' books frequently fail to develop the plot further, merely keeping things afloat until the final book.
Yet as you may have noticed, I still rated this book lower than the previous one. There is only a single reason for this; the ending is boring. The twist that sets the endgame in motion has been done to death and the motivations behind it are tentative at best.
Solid enough, but I do worry a bit that the series is running out of steam plot wise.