Hamnet is Shakespeare's son who died aged 11, but the book is not really about him, it's about his mother Anne, or Agnes as she is known in the novel. It begins in 1596 with the boy home alone. The narrative follows him through the house, setting the scene, introducing us to the players. In the second chapter we go back in time to 1582 or thereabouts and meet Agnes Hathaway and her family.
For about a third of the book, Part I, the story flips back and forth, alternately following the action in these two years when Hamnet became ill and when Agnes and Shakespeare married. Part II deals with the aftermath of Hamnet's death. It's told in the present tense, which works especially well in the first chapter when Hamnet is running breathlessly and anxiously around town. It didn't feel right tho' in Part II when the family is grieving. The present tense is too impersonal, too abrupt and insensitive for the emotions portrayed.
At the end of Hamnet Maggie O'Farrell notes "this novel is the result of my idle speculation", which one hopes is profitable for the author but is certainly fortunate for those who enjoy a historical novel with a bit of romance and a strong female lead.