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?