Action! Adventure! Discoveries and alliance before the outbreak of war! In a post-apocalyptic steampunk California!!
While I think the story stands on its own, it does have a lot of plot spoilers for The City of the Founders.
For instance, the opening sequence is Romulus going up into the mountains to investigate a shipwreck, because he had been told something in the last book and could verify it there. A venture enlivened by weather and sabretooths, by Max's coming to his aid against his orders, by the strange location where he finds refuge for them. (There are a few more hints of what the apocalypse was in this book, but not much.)
And then he returns for a council of war, as they group against the Founders, who have manifested such hostility and marshaled such forces that they have no hope except in a grand alliance. A late arrival to the discussion observes that by coming, they have cast their lots: Founders' spies will have reported them by now.
The story goes on, covering a ball, nightmares, a heroic effort to deliver a message, a battle, the need to stop the lynching of a spy, insoluble equations that would make immortal the one that solved them, and much more.