The Light of Western Stars by Zane Grey

In waking and sleeping hours Madeline Hammond could not release herself from the thralling memory of that tragedy. She was haunted by Monty Price's terrible smile. Only in action of some kind could she escape; and to that end she worked, she walked and rode. She even overcame a strong feeling, which she feared was unreasonable disgust, for the Mexican girl Bonita, who lay ill at the ranch, bruised and feverish, in need of skilful nursing.

Madeline felt there was something inscrutable changing her soul. That strife -- the struggle to decide her destiny for East or West -- held still further aloof. She was never spiritually alone. There was a step on her trail. Indoors she was oppressed. She required the open -- the light and wind, the sight of endless slope, the sounds of corral and pond and field, physical things, natural things.