Riders of the Purple Sage. by Zane Grey

In a small Mormon community in southern Utah, Jane Withersteen, a young, unmarried Mormon woman faces growing pressure to marry a local elder of her church. Elder Tull, a polygamist, already has two wives and seeks to marry Jane not just for her beauty, but to take control of the ranch her late father passed on to her. Jane's resistance to marriage only serves to increase the mounting resentment against "Gentiles" (non-Mormons) in the area.
Bern Venters, one of Jane Withersteen's ranch hands and potential suitor, becomes the focus of this resentment and is nearly killed by Elder Tull and his men before a mysterious rider interrupts the procedure. The rider, a man named Lassiter, is a gunslinger known for his exploits in other Mormon settlements further north. Lassiter's intercession on Venters' behalf sets off a chain reaction of threats, violence, theft, and murder as Jane Withersteen fights to maintain both her ranch and her independence.
First published in 1912, Riders of the Purple Sage is considered to have played a prominent role in shaping the Western genre. It was Zane Grey's best-selling book and has remained popular ever since.