This is Jon Krakauer's book about the Mormon religion and specifically the radical FLDS offshoot. Subtitled A Story of Violent Faith.
This stuff - specifically, the disadvantaged position of women in the FLDS society - fascinates me the same way I'm fascinated by Victorian marriage horror-stories. (Tenant of Wildfell Hall, Daniel Deronda.) Marriage, in a society where men have a disproportionate share of the power, can so easily devolve into a nightmare for women. And in an era where women didn't really have other options for supporting themselves, the stakes for choosing a partner and/or falling in love were incredibly high. I'd like to see a bit more of that awareness in historical romance.
Under the Banner narrates several instances of women whose husbands converted to FLDS ideals and became convinced it was their religious duty to take additional wives. Good God. After marrying in the church, and agreeing to the "forsaking all others" bit. How does any woman come to terms with that?