Leviathan Falls is the ninth, and I believe final, book in the Expanse series. We start with our intrepid heroes running out of places to hide from the Laconian military.  The poor Rociante doesn’t just have the leader of the Resistance onboard, but also the daughter of the head of the empire [by choice]. They start loosing resources, people, and things are pretty bleak.

Without exposing too much, the leader of the Laconian Empire, Winston Duarte, who was essentially catatonic at the end of the last book, wakes up and disappears into space.  Everyone is looking for him and our friends onboard the Rociante.  Slowly everyone starts to realize what Duarte is up to, and no one is down with it.  Enemies are forced to work together.

In the earlier books there is discussion about the makers of the ring gates, and how they fought against some mysterious enemy.  And this same “enemy” seems to be fighting humanity in isolated incidents.  But Leviathan Falls kind of drops that angle and focusses on Duarte’s plans for humanity using the power of the Ring Station.  They could have devoted another book or two to these beings and their grievance (which seems to be the effect of the Ring system on them).

if you get why Casablanca is a great film, you’ll appreciate the ending of the series.