I Am Pilgrim by Terry Hayes

A young woman is found dead in a seedy New York hotel room, her body dissolved by acid and her identity erased. The method is drawn from a book written years earlier by a man known only as Pilgrim—a brilliant intelligence operative who vanished into anonymity. But when a ruthless terrorist known as the Saracen emerges from the deserts of Saudi Arabia, intent on unleashing a catastrophic biological attack, Pilgrim is forced back into the shadows he tried to leave behind. Across continents and decades, two men—one driven by vengeance, the other by duty—race toward an inevitable confrontation that could alter the fate of humanity.