Right as Rain - George P. Pelecanos

nDerek Strange is a black ex-cop in Washington D.C. who now makes a living running his own private detective agency. He is hired to investigate the killing of an off-duty black policeman by a white police officer -- a killing that was supposedly accidental, but that has opened difficult questions about racism on the force. In the course of that investigation the white officer, Terry Quinn, becomes Strange's friend and then his partner. Together they try to uncover what really happened that night, when Quinn came upon a confusing and treacherous crime scene. Along the way they confront the kingpins of a flourishing drug trade and some of the most implacable, dead-eyed killers ever to grace the pages of a novel.