Half-Broke Horses is a novel based on true events from the author's grandmother's life. In other words, it is a historical novel. Jeannette Walls herself uses the term "novel" to describe the book in her author's note at the end of the book: "[S]ince I don’t have the words from Lily herself, and since I have also drawn on my imagination to fill in details that are hazy or missing [...] the only honest thing to do is call the book a novel."
The events that occur in the novel did really happen to Walls's grandmother, Lily Casey Smith, and Walls did substantial historical and familial research so that the plot points would be historically accurate. However, the term "novel" is also accurate because of the creative liberties Walls takes with the dialogue and narrative. She cannot truly know every sentence that her grandmother said or thought, and that is not the point of the novel. Instead, she wanted to create a fictionalized story to preserve her grandmother's memories.
Half-Broke Horses serves as a prequel to The Glass Castle, Walls's memoir of growing up traveling with her parents Rosemary (later Rose Mary) and Rex, although it was published after The Glass Castle. Although the books are connected, they are completely different in genre. The Glass Castle draws from Walls's own experiences and is a contemporary autobiography, while Half-Broke Horses is a historical reimagining of Lily's life.