Devil's Bargain (Red Letter Days #1)

Jasmine "Jazz" Callender is on the downhill slide to ruin. Once a decorated homicide detective, she's lost it all: her former partner's been convicted of murder, she's been cashiered out, and she's drinking away what little self-respect she's got left. But Jazz has a talent for trouble, and somebody knows it. When a mysterious, sexy stranger comes looking for her with a fateful red envelope in his hand, she's about to make the deal of her life...for good or bad.

The deal requires her to enter into a partnership with a stranger and investigate cases that arrive in special red envelopes...which is odd enough, but gets weirder as she and her new partner Lucia realize that they may be working for someone with supernatural abilities.

And maybe they're not on the side of the angels anymore.