This novel (and presumably series) has many of the elements of urban fantasy, but it's set some time in the future, making it an urban fantasy/cyberpunk hybrid.
Dante is one of the most powerful necromances (of course) and one day a demon shows up on her door and tells her the Prince, a.k.a. Satan, has a job for her and it's not optional. She has to meet the Prince in Hell, and he gives her the job of recovering an egg from a demon that escaped hell, and who happens to be an old nemesis of hers. To aid and protect her he gives her a demon familiar (the same demon who showed up on her doorstep).
Saintcrow has created an interesting world. The technology feels more cyberpunk than scifi. For example there are sliceboards, which are sort of like flying skateboards. In terms of government, the Mob is a legally recognized entity which reminds me of Neal Stephenson's Snow Crash, which is a cyberpunk novel.
There are several different schools of magic (I think 8) although they seem to be called both psychic and magical powers. Each school has its own powers and strengths. It seems that psions can have powers from more than one school, (Dante, for example, has all the powers of a necromance and also a slight gift of foresight, which is from another school of magic). There's no indication that psions can have an equal level of power from two or more schools, however.
If there was any explanation for why psions "came out" so to speak, I don't remember it. Their origins are explained in the course of the book, but they aren't generally known.
The history is somewhat murky. Apparently psions and demons weren't always living openly. The appearance of demons destroyed the vast majority of people's faith in Christianity. It's never really explained why though. Maybe it's because the other side never makes itself known. It may be because demons claim they aren't fallen angels, but demons lie. Besides, some of the other mythology is the same. There's still the level of demons that falls in love with the daughters of man (in Christian mythology I believe it's the Grigori) and that brings ruin on both the women, the offspring and the demons. And demons that fall in love with mortals are called Fallen. There's absolutely no mention of how followers of other faiths reacted to all this.
There are ways in which this feels like a first novel, though apparently it's not. The history and mythology isn't clearly defined. I have no idea why they're called necromance instead of the more common necromancer. Dante also has an extremely annoying habit of thinking something and then repeating the thought aloud. If she's going to say it, she doesn't have to think it first.
Still, despite the flaws I enjoyed the book and will continue to read the series.