Strange New World — Vivian Shaw

Strange New World finds Greta Helsing, doctor to the undead, facing the latest and strangest challenge of her career…accompanying an anxious angel and a sullen demon on a road trip across America in the conclusion to a delightful fantasy series.
 
After narrowly avoiding the end of the world, the leaders of Heaven and Hell are struggling to collaborate according to the terms of their new treaty -- especially because angels and demons are, quite literally, allergic to each other. Seeking a solution, the powers that be decide to see if the allergy persists on Earth by sending an angel and demon on a research trip, first stop: New York City. And what better chaperone than Dr. Greta Helsing, who happens to owe Hell a few favors of her own?
 
But there's unrest in New York's monster underworld and Greta and her team are about to land in the middle of it. Something is off in Heaven and on Earth, and Greta will have to figure out just what that is if she hopes to protect those she loves most.