B. M. Bower 
Jean of the Lazy A 

This was B. M. Bower’s 15th novel, and like her The Phantom Herd a year later, it draws on her knowledge of the movie business. Sixteen-year-old Jean Douglas, the title character, is a no-nonsense daughter of a Montana rancher, Aleck Douglas, who in the opening chapters is wrongly found guilty of murder and sent to prison. With the help of a ranch hand, Lite Avery, she spends the rest of the novel finding the real killer. (Goodreads)