Right as Rain — George P. Pelecanos

A YOUNG POLICEMAN IS DEAD and the bullet that killed him came from another policeman's gun. Derek Strange, a former cop who now runs his own private detective agency, is hired to investigate: the mother of the victim wants Strange to clear up lingering doubts surrounding her son's death. Terry Quinn, who pulled the trigger, has been exonerated in the official inquiry, but his guilt torments him. He joins Strange in the investigation and begins to examine the ways in which his own prejudices might have clouded his judo-tent. Strange and Quinn seek answers in the darkest sectors of Washington, D.C., where racism and ruthless capitalism spawn the sort of lawlessness that can taint even the most upright and honest men.