Final answer:
Billie Jo's spirits are sustained by her passion for music and her resilient nature, which help her to cope with the difficulties of the Dust Bowl and personal loss.
Step-by-step explanation:
What sustains Billie Jo's spirits in the novel is her love for music and her resilience. Despite the hardships and the devastating events she encounters, including the loss of her mother and the difficulties of living in the Dust Bowl during the Great Depression, Billie Jo finds solace and strength in playing the piano. It's her passion for music that serves as a beacon of hope, allowing her to express her emotions and endure through the tough times. Furthermore, her determined spirit and inner strength are significant factors in helping her to confront challenges and move forward in life.