Oracle of Philadelphia: Book One of the Earthbound Angels Series in Bloomington, MN

Carrie works at a diner in South Philadelphia, dispensing advice to humans and angels wise enough to seek her counsel. But there are some problems that even the best advice can't solve.
Her latest supplicant, Sebastian, is unique among those who have sought her aid. He sold his soul to a demon in exchange for his sister's life, but his heart remains pure.
Carrie has lived for millennia with the knowledge that her immortality is due to the suffering of others, and she cannot bear to see another good man damned when it is within her power to prevent it.
In order to renegotiate his contract, Carrie must travel into the depths of hell and parley with the demons that control its pathways. As the cost of her journey rises, Carrie must determine how much she is willing to sacrifice to save one good soul.