What are the motifs in Riders of the Purple Sage by Zane Grey?

Zane Grey confronts the zealous belief-system used by nineteenth-century Mormon settlers to control their multiple wives in Riders of the Purple Sage. Through Lassiter's voice, Grey attacks the brain-washing and intimidation tactics employed against Mormon women of the era, particularly Milly Erne. Milly is not Mormon by birth or inclination; she is kidnapped from her marital home by three Mormon men and coerced into becoming one of the many wives of rancher Withersteen. Milly is able to send a letter to her true husband shortly after her disappearance, and in this first letter Milly is desperate to be rescued and wants, more than anything, to reunite with her husband and the father of her child. Milly gives birth to her only child in captivity, having been stolen from her home while pregnant. Several years pass without further word from Milly before her second and final letter arrives.