Carl Novakovich 
The Tomb [EPUB ebook] 

John Gideon, a former homicide detective turned P.I., has dug too deep. Now, he has to come to terms with the truth of the world, while searching for the only family he has left.
Together with his new partner Beth May – a spell-wielding demon who has turned her back on Hell – the two are humanity’s last hope to stop a collective of Fallen Angels, known as The Watchers, from breaking the first of the Seven Seals of Revelation.
If the Seals are broken, they will set free sickness, war, and death on the world, until there is nothing left but an apocalyptic wasteland. John and Beth have the tools needed and the strength to stop them. But are they already too late?