Donna Tartt - Small friend

It all begins with the disappearance of nine year old boy, Robert Clive-Dufresne. Missing child found hanging from a bough of a tree in the yard of his own house. The killer did not leave traces, and the investigation comes to a standstill. After ten years of the boy's sister, who at the time of the murder was only two years, he decides to find and punish the guilty. It "computes" the criminal and contemplating revenge.