Jean of the Lazy A by B. M. Bower

Classic western. According to Wikipedia: Bertha Muzzy Sinclair or Sinclair-Cowan, née Muzzy (November 15, 1871 – July 23, 1940), best known by her pseudonym B. M. Bower, was an American novelist who wrote fictional stories about the American Old West. Born Bertha Muzzy in Cleveland, Minnesota, she was married three times: to Clayton Bower, in 1890; to Bertrand W. Sinclair, in 1912; and to Robert Elsworth Cowan, in 1921. She wrote 57 Western novels, several of which were turned into films.