The Stone Heart is the second book in the Nameless City trilogy from Faith Erin Hicks.
Kaidu and Rat have only just recovered from the assassination attempt on the General of All Blades when more chaos breaks loose in the Nameless City: deep conflicts within the Dao nation are making it impossible to find a political solution for the disputed territory of the City itself.
To complicate things further, Kaidu is fairly certain he's stumbled on a formula for the lost weapon of the mysterious founders of the City. . . . But sharing it with the Dao military would be a complete betrayal of his friendship with Rat. Can Kai find the right solution before the Dao find themselves at war?