Kanishk Tharoor goes on the murky trail of the missing Genie of Nimrud – a huge, 3,000-year-old carved figure that once protected a palace; the Winged-Bull of Nineveh, an Assyrian sculpture that guarded the gates of one of the most fabled cities in antiquity; and a looted Sumerian seal stolen in the aftermath of the 2003 invasion of Iraq.