Last Round. The Erin O'Reilly Mysteries, #30

Make every shot countAfter dealing with gangsters, terrorists, and crooked cops, Erin O'Reilly isn't too worried about her latest enemy. Would-be mobster Richard O'Malley is doing his best to fill his dad's shoes, but he'll never be anything but a small-caliber hoodlum. However, even a tiny bullet can be lethal, and Erin knows Richie wants to kill her, so she's keeping a close eye on him. But the surveillance goes horribly wrong.
An assassin guns down Richie's family in broad daylight with a gun belonging to Erin. Multiple witnesses identify Erin as the killer. Instead of investigating the crime, she is now the prime suspect. Suspended from duty, stripped of her authority, Erin knows she's the target of a sadistic scheme. As she and her faithful K-9 Rolf scramble behind the scenes to disarm their mysterious enemy, she must fight not only killers but the NYPD itself.
Revenge and loyalty collide in a final showdown pitting an unlikely alliance of rogue cops and reformed criminals against a brilliant and deadly opponent.