According to the back story of this series 40 years ago a genetically engineered virus was released which kills half of the worlds population and exposes to the world the creatures that had been living in the shadows alongside humanity, vampires, werewolves, witches, demons etc.
Our hero Rachel Morgan is a witch who works for an agency who hunt down this night creatures when they break the law.
Morgan hates this job and sets out to make her own way by running her own business as a freelancer, using her skills to make a living, and staying alive as various supernatural nasties chase after her.
This is really chick lit with monsters, but Harrison has a light touch that makes it work, and fun to read. Sure sassy females using their wits and a little luck to survive are not uncommon in fiction, but Morgan is a good addition to their ranks.