The premise for the Expanse series is it's roughly two hundred years in the future and mankind has spread to the rest of the solar system. Earthlings live on an overpopulated and resource stretched planet but in relative comfort, Martians live in a militarized communist society, and the Belters live in abject poverty in order to serve the needs of the two settled planets. Our heroes actually start in a pretty good place, however. The protomolecule which was the focus of the first volume has been neutralized, the Rocinante crew has a job as pirate hunters, and neither Earth nor Mars is particularly interested in wiping out the other. This all goes to hell, though, when Ganymede Station is attacked by an unknown force that wipes out a group of Martian Marines.
Caliban's War introduces some interesting new characters with Bobbie Draper, a beautiful but physically intimidating Marine, who suffers from survivor's guilt after the aforementioned attack as well as Avarasala, the foul-mouthed spymaster of the U.N. Both characters are every bit, if not more entertaining than the original protagonists of Leviathan's Wake. I really enjoyed Avarsala's introduction where she finds out Holden is in hiding by using...a fake beard. Her incredulity at Holden thinking that would work is hilarious.
The crew of the Rocinante actually gets a lot of character development in this book too. Holden is coping with the loss of his naivite and idealism. Naomi is dealing with the fact Holden isn't quite the man she started dating. We also get a good amount of new information regarding Amos as well as Alex. Amos is one of my favorite characters in the Expanse and this book highlights being from Earth doesn't mean you're not from a hellish childhood surrounded by poverty. I loved their sections of the book and how they brought the perspectives of their respective planets to the forefront. Is Detective Miller missed? Incredibly. He was one of my favorite parts of the original series and his absence is notable. However, that doesn't mean I didn't enjoy this book a great deal.
However, there is a number of problems with the book which I think hurt its storyline overall. The first of these is the fact this feels a bit like a video game plot. Someone is using the protomolecule to make super-soldiers ala Resident Evil or the Alien movies. This, despite the fact the protomolecule is completely beyond anything human technology can command. We're not talking centuries beyond human technology but billions of years so the idea it can be used to make monsters under the control of Jules Pierre Mao is ridiculous.
I also wasn't a big fan of Praxis, the scientist whose missing daughter is the impetuous for the plot. He's an okay character but doesn't really have any layers other than, "Find daughter" and "grow space crops." Even so, it's nice to get a sense of the Rocinante crew by the fact when he comes to them and asks them to help him find his probably-dead child, they immediately drop everything and help him try to find her.
In conclusion, Caliban's War is a decent continuation of the Expanse novels even if I felt it was a bit derivative of other sci-fi franchises. I would have been more interested in continued focus on the Mars-Earth-Belter relationships versus more study of the protomolecule. The new characters more than make up for this, though, as does expanding on the existing heroes. Too many books don't continue expanding their leads in sequels, just repeating the behavior of previous works. That isn't the case here.