The Triple Death of the Third Character

After spending a long time in prison, a man is pursued by international agents who want to kill him. Without knowing exactly what is going on, the protagonist realizes that they want to kill him because the book he wrote about his time in prison contains secret codes that should not have been revealed. He, who did it by chance, realizes that he is in grave danger. One by one, the characters in the novel are eliminated so that the mysterious secret remains hidden forever.