The Heartless One by Emma Hamm

When Princess Jessamine’s kingdom is overrun by an incurable plague, she does her duty and enters a political marriage to save her people. Too bad her new husband’s first act is to slit her throat and seize the throne.

But death isn’t the end for Jessamine. Her spirit is met by the Deathless One, a god of death yearning to return to the mortal plane -- and he needs her help to resurrect him. He senses powerful magic in her frail and broken form, so the two of them make a deal: her life and the return of her kingdom in exchange for his restoration.

Deals with gods are never so straightforward. Deathless One is a dangerous ally-- but the longer they spend together, the more Jessamine is drawn to him. Perhaps the more appealing throne is the one by his side...