"Thunderhead" by Mary O'Hara, was the follow up to which of these novels?

Mary O’Hara (July 10, 1885 – October 14, 1980; born Mary O’Hara Alsop) was an American author, screenwriter, and composer, best known for the horse story for all ages, 'My Friend Flicka'. Born in Cape May, New Jersey, she was raised in the Brooklyn Heights, New York, mainly by her father. Her mother died when she was a child.

She married her third cousin, Kent Kane Parrot, in 1905 against her father's wishes. They had a daughter, O'Hara Parrot, born in 1908, who died of skin cancer in her early teens, and a son, Kay (Ken) Parrot (born in 1910).

Following the end of her marriage to Parrot, Mary O'Hara worked as a Hollywood screenwriter during the silent film era. Her screenwriting credits included the movies 'The Last Card' (1921), 'The Prisoner of Zenda' (1922), 'Braveheart' (1925), and 'Framed' (1927).

The ranch life and the rugged Wyoming backdrop proved to be the inspiration for what would become Mary’s most enduring work. 'My Friend Flicka', the story of Ken McLaughlin, a rancher’s son and his horse, was published in 1941, and became a trilogy along with 'Thunderhead' (1943), and 'Green Grass of Wyoming' (1946). In the second book of the "Flicka" trilogy, 'Thunderhead', Ken is determined to break Flicka's rebellious colt, Thunderhead, to get the racehorse of his dreams. But the attempt proves difficult, and in the process Ken learns a great deal about himself.