In 1596, William Shakespeare’s 11-year-old son Hamnet died in Stratford-upon-Avon. Four or so years later, Shakespeare wrote the play considered by many to be his greatest work, giving its tragic hero a variation of his dead son’s name. Almost four centuries later still, Maggie O’Farrell was studying Hamlet at school and learned of the boy Hamnet, whose life has been little more than a footnote...