M Is for Malice by Sue Grafton

Private investigator Kinsey Millhone is hired to track down a long-lost heir, Guy Malek, who vanished nearly twenty years ago after being disowned by his wealthy family. When Kinsey locates him, Guy has reformed his life—but his return home stirs old resentments and buried secrets. Within days, one of the brothers is murdered, and Kinsey finds herself entangled in a web of greed, guilt, and revenge. As she digs deeper, every truth she uncovers exposes a darker betrayal, and the line between justice and vengeance begins to blur.