Dead Witch Walking, by Kim Harrison

I started getting lost on some of the details of this series as I was reading #7 and #8, so I decided to go back to the beginning and catch back up.
The alternate history behind this series is that in the late twentieth century, a rogue biological agent got loose from the labs, infecting tomatos, which wiped out a great portion of the human race. During this time period,  called The Turn, the witches, weres, vampires and other supernaturals who had been a very small and hidden part of the population all along were not affected by the plague, and they emerged from hiding to make sure key industries and services continued. When the plague was over, a somewhat uneasy peace developed, as humans decided to forego hunting and killing the Inderlanders - the supernatural races - as they used to do, Pre-Turn, whether out of gratitude or mere pragmatism is left to your imagination.
Rachel Morgan is a witch, employed by Inderlander Security, or IS,  as a runner. Her mission is to hunt down supernaturals who are using their abilities to harm or take advantage of either humans or other Inderlanders. Her runs have been going amiss lately, and she finally decides it's time to leave IS. When she gives her notice, IS issues a death warrant on her - IS is a bit like the Mafia in that few leave their contracts and survive. Joining her departure from IS is another runner, Ivy, who is also a living vampire - a vampire created by being born to another vampire, rather than by blood exchange conversion - and Jenks, a pixie.
They join forces to form their own private investigations business, and they all move into an abandoned church, which they use as both living quarters and offices. The rest of the story deals with how Rachel avoids the hit squads of the IS, scores a big bust on a distributor of illegal engineered biologicals, and ends up owing a demon a favor. We get introduced to some of the key characters in the rest of the series, like Trent Kalamack, the elf politician, Glenn Edden, the FIB agent, and Nick Sparagmos, the wizard and thief. Rachel's tendency to set out on missions impulsively, without any contingency planning, gets her into all sorts of dicey situations, which are really the backbone of this series.