It's set in a steampunk alternate England, nine years after the fall of the two-century regime of the Horde--yup, the Mongolians have nanotech in this world, and they were ruthless about using it, infecting and controlling the population of Britain with radio-operated "bugs" that suppress the emotions (except when whipped up into an occasional mating frenzy, the issue of which were taken from their parents and raised in centralized crèches). They also cyborged (not that that's a verb) thousands of laborers: pneumatic hammers for miners' legs, arms modified into sewing machines--reminiscent of Mieville's Remade, though less Dantesque.
After the former pirate Rhys Trahaearn destroyed the Horde's control tower, acquiring the titular dukedom for his trouble, the slowly reconstituting society finds itself divided between "buggers," the nanoagent-carrying former subjects, and "bounders"--those with ancestors wealthy or lucky enough to have escaped to the New World before the occupation, now returned to a country they still consider their own (yes, I love that these terms are repurposed insults!). Relations between the two groups are strained at best--bounders are obviously resented by those who suffered under Horde control, and they in their turn distrust buggers who can be controlled by technology. And bounders and buggers alike often view our heroine, police inspector Mina Wentworth, with disgust or hostility: she is half-Horde, the product of a Frenzy at which her mother was raped. (Her mother, in fact, gouged out her own eyes upon seeing her daughter's telltale features. Though she seems to have come to terms with it over the 30 years of Mina's life, and has awesome new prosthetic eyes.)
Mina meets the Iron Duke (to what extent that moniker is literal I shall leave as an exercise for the reader) investigating a dead body found on his estate--a man who seems to have dropped from the sky. They have the expected Sudden Overwhelming Hot Pants for each other, but their mystery-solving airship-voyaging ass-kicking boot-knocking journey to Happily Ever After is fraught with some really dark and complex issues: not only is Mina the product of rape, she was forced into a Frenzy shortly before the fall of the Horde and equates her own sexual desire with loss of identity and control, leading to a really excruciating scene where Rhys mistakes her struggling during oral sex as arousal, and essentially assaults her by accident. I KNOW, RIGHT?!?! And it's a believable scene, too, not the now-squick-inducing "forced orgasm" trope found in 70s romance novels (ever so pithily summed up by the Smart Bitches as "rapetastic"); his shock and horror at realizing what he's done are backed up by his own history, sold into prostitution at the age of eight.
Again: I KNOW, RIGHT? This is not a fluffy book, and I have to applaud its audacity.
(Also? I am pretty sure there's some kind of 9/11 allegory going on, though I can't really tease out direct equivalents. But it takes place nine years after the fall of a tower, and sometimes the buggers are referenced as being sleeper agents for a foreign power, echoing some anti-Muslim rhetoric...I might be COMPLETELY reading too much into it, though.)