Caliban’s War is the second in the Expanse series of book, following Leviathan Wakes.

This installment starts with a shooting war breaking out between Earth and Mars on the farming colony of Ganymede.  Both sides think the other started it, but the answer gets more complicated as intelligence communities investigate the details.  We are introduced to three new characters: Bobbie, a Martian Marine who survives the attack on Ganymede, Prax, a botanist on the moon whose daughter is kidnapped right before the attack, and Avasarala, a powerful UN official who takes command of any room she enters.  Prax meets the out intrepid crew of the Rociante and they agree to find his daughter, only to find this overlaps with finding the people who have been weaponizing the “protomolecule” that initiated the entire storyline.

The book delves into conflict between captain James Holden and hist first officer Naomi Nagata.  In particular whether Holden is losing sight of his humanity as the number of people he has killed increases.  Has shooting people become his only tool?  Lots of parallels are made to the late detective Miller they met in the first novel.

As you might expect, the book ends with multiple fleets descending on a moon containing the main base for weaponizing the protomolecule, and no one knowing who to trust.