Book 3 in the alternate universe Black Knight Agency series.
"After Jane Doe and Steven Quinn retired, they expected the Underworld to just move on, another organisation to take over, the holes to be filled. Unfortunately, that is not what happened, so when a murder happens among the drug lords and nobody knows how to solve that problem, they are called in again. While Jane works her way through the case, she also picks up the slack in other areas, restoring some balance, greasing some formerly un-greased palms to ease things back in place. There’s a grey eminence needed to keep the Underworld in balance and both fate and her boss have made a decision."