The Sixth Demon by R. A. Steffan

An angel locked out of Heaven. A demon on the hunt for an escaped soul.
Watch out, because all Hell is about to break loose.
When the Archangels slammed the pearly gates shut on the great war between the Demons and the Fae, the angel Shemasiel missed the memo. Trapped alone on Earth, she’s spent the past few millennia sticking her nose in where it’s not wanted, and generally being a nuisance to the people in charge.
Posing as human investigative reporter Neveah Lane, she’s first on the scene of a mysterious death with hallmarks of the supernatural. But when one of Hell’s most notorious demons is also sent to investigate, it quickly becomes apparent that more is at stake than the death of a single woman.
Now, with the truce between two powerful, warring races hanging in the balance, a stranded angel and Hell’s most devious spymaster must choose whether to oppose each other on principle, or work together for peace. As the two orbit each other in pursuit of their competing goals, Neveah finds herself experiencing the appeal of the forbidden for the first time in her ageless existence.
Angels are supposed to be immune from temptation. Too bad Neveah was never a very good angel.