Season of Skulls (Laundry Files Series #12) by Charles Stross

Season of Skulls continues Hugo Award-winning author Charles Stross's Lovecraftian Laundry Files series.

Welcome to the sunlit uplands of the 21st century! Britain's avuncular Prime Minister is an ancient eldritch god of unimaginable power. Crime is plummeting as almost every offense is punishable by death. And everywhere you look, there are people with strange powers, some of which they can control, and some, not so much.

Hyperorganized and formidable, Eve Starkey defeated her boss, the louche magical adept and billionaire Rupert de Montfort Bigge, in a supernatural duel to the death. At least, she has reason to hope he's dead. But though she's now in charge of the Bigge Corporation, she's not free of him yet. Through the fecklessness of her brother Imp, combined with the intricate feudal law of a tiny Channel Island, it would appear that unbeknownst to her, she was married to Bigge--and that proving his death and releasing herself from his arcane bindings will take years and cost millions.

Then an emissary of the Prime Minister arrives with an offer that she absolutely can't...well, you know.

This is the final novel in the trilogy that began with Dead Lies Dreaming and continued with Quantum of Nightmares.