The Last Detective by Robert Crais

Elvis Cole's relationship with attorney Lucy Chenier is strained. When she moved from Louisiana to join Elvis in Los Angeles, she never dreamed that violence would so easily touch her life -- but then the unthinkable happens. While Lucy is away on business and her ten-year-old son, Ben, is staying with Elvis, Ben disappears without a trace. Desperate to believe that the boy has run away, evidence soon mounts to suggest a much darker scenario.