Sacrifice — Andrew H. Vachss

A battered child has vanished. The trail leads down, maybe as low as Burke has ever gone, into the sleazy subbasement of morality. He's not hunting for a man but for evil in the heart of man. It takes a secret meeting with an urban voodoo queen to send Burke to the doorstep of unforgivable sin-a cult whose horrific acts of abuse have turned a child savage. Burke wants the adults to pay, wants to see it happen, wants to make it happen....