The world-building is fascinating. Demons seduce or trick or steal souls from humans, which solidify as coins, the person's face on one side and true name on the other. The coins are used as currency. Angels protect humans but only take souls at death. If humans avoid either fate, and possibly also because of moral implications (unclear), they can choose the Third Way, where demons and angels are not allowed to go.
Incubus Kenan discovers an angel locked in a cage and buys her. She's come to Hellsgate to try to retrieve the soul of her charge from a hellhound. Kenan wants her soul -- the soul of angel is a challenge. Their attempts to find the hellhound and bargain for the human's soul become entangled with a conspiracy to start the Apocalypse.
The romance is nice, and there's some nice evasion of/turnabout of the usual tropes of angelic innocence (the angel Jahel is curious about sex, enjoys it, feels no bodily or moral shame about it). I wish it had been longer -- the relationship arc feels complete, but I would have wished for more distrustful attraction in the beginning, just because I like that kind of thing. More objectively, the bigger conspiracy plot feels shoehorned in and rushed -- it should have been expanded or left out.