Heart of Iron by Bec McMaster

In Victorian London, if you’re not a blue blood of the Echelon then you’re nothing at all. The Great Houses rule the city with an iron fist, imposing their strict “blood taxes’ on the nation, and the Queen is merely a puppet on a string…

Lena Todd makes the perfect spy. Nobody suspects the flirtatious debutante could be a sympathizer for the humanist movement haunting London’s vicious blue blood elite. Not even the ruthless Will Carver, the one man she can’t twist around her little finger, and the one man whose kiss she can’t forget…
Stricken with the loupe and considered little more than a slave-without-a-collar to the blue bloods, Will wants nothing to do with the Echelon or the dangerous beauty who drives him to the very edge of control. But when he finds a coded letter on Lena-a code that matches one he saw on a fire-bombing suspect-he realizes she’s in trouble. To protect her, he must seduce the truth from her.

With London on the brink of revolution, Lena and Will must race against time-and an automaton army-to stop the humanist plot before it’s too late. But as they fight to save a city, the greatest danger might just be to their hearts…