What is not to love about this book? There's a nice mix of dark, gritty occult horror setting (think: Supernatural), driving police-procedural story (think: True Detective) and a competent, assertive, driven, and kick-ass main character (think: a witchy Dr Temperance Brennan) beating the hell out of demons to protect the innocent. In particular, the world building and characterization are fantastic - particularly in the audiobook, where the spot-on performance gives a massive boost to at least one crucial character (for I do declare, who couldn't love a certain well-spoken demonic gentleman when his words are sugar-coated in such a darlin' n'awleens accent?)
In full disclosure, I've listened to most of the Danial Faust books. You should too - at least to discount them - before starting on this, because there are immediate late-game spoilers for that series in the opening of this one. This series exists in the same world (God and Heaven are MIA, and we're all futilely slugging it out with Damnation until we end up there anyway), but drops the main problem with those books, which is Daniel Faust himself (a cowardly, psychotically selfish, sex-starved traitorous monster of a man whom Audible's profanity-filter for reviews prevents me from adequately describing here) in favor of the altogether more likeable Harmony Black. As my enjoyment of the Faust books was ruined by my growing hatred of the main character, having a protagonist who actually fights the forces of hell rather than just sleeping with them is an enormous bonus, and makes the world enjoyable for me again. That Harmony knows the fight is futile and knows the enemy is winning, but fights anyway, because no-one else is going to step up to do it otherwise is an enormously compelling basis for her character, and really gets you to care about her and her fight.
In terms of how that fighting is done, although Harmony Black is a witch, her biggest assets are her investigative rather than magical powers. Mad pyscho pyromaniac fireball-spamming wizards do exist in this world - and Harmony can throw down with them in a pinch - but her powers are explicitly more 'leveling the playing field' than 'leveling the building'. In almost all circumstances, Harmony uses the stopping power of her service-issue handgun over that of a lightning bolt (quicker, easier to do the paperwork for, and less tiring) and that gives the story a much more believable edge, as magic becomes the counter to specific problems, rather than the resolution to absolutely everything.
This is very much Harmony's book (and my one criticism would be that the book does absolutely everything it can to ram that down your throat, by having her personally invested in the case - pretty much to excess - rather than this just being another monster to kill) and that works to it's advantage so early in the series, as it gives us a real grounding in her story and motivations. It also really pays off in the ending, by giving her some closure on her traumatic childhood, and adding some real depth to her personality. For myself, I was really interested in a lot of the other characters, and to some extent Harmony's prominence steals the limelight from them - but I'm happy that development is coming later (we can see the setup for at least two of their stories in this book).
All in all, I'd heartily recommend this for anyone who likes the idea of a monster-killing FBI detective, or (like me) liked the Daniel Faust books while hating Daniel Faust. In general though, you should give this a shot whatever you like, because there is honestly enough in here to grab anyone and keep them coming back for more.