In the future world where Dante lives, psychic powers and magic and very real things and links between Earth and the dimension usually called Hell are not uncommon. There is a strong hint that the demons have been influencing human evolution and that Lucifer has something in mind that involves Dante and the daughter of her dead friend.
This book returns Dante to her roots in Saint City, but the best part of these roots have been ripped out by the murders of her two closest remaining friends, and she swears to avenge them.
None of these problems are easily resolved, and we are left with a literal cliffhanger on the final page of the novel, which will be resolved in the fifth and final book of the series. In effect, Saint Crow has written a five part novel. If I weren’t intrigued by the results, I’d been peeved by the trick.