When a man kills a woman and her toddler in cold blood and claims god told him to do it, your first reaction is he must be crazy. Because only a crazy person would do that, right? Actually, he's quite rational under the construct of his belief system. The biggest and most disturbing takeaway from this book is that those who murder innocent people, exploit women and molest children in the name of god are not mentally ill, but rather responding to their circumstance in a logical manner using their beliefs and doctrine to guide them.
Wait a minute, wasn't this a book about the LDS church and spin off fundamentalist sects? Yes! The book intersperses Mormon history with a true crime account of the events before, during and after the above mentioned murder. It's an interesting approach and helps illustrate the circumstances in which this could happen. While this is ostensibly about the LDS church, it delves into that fuzzy line that divides garden variety god-fearing people from religious fanatics.
Disclaimer - I am not a Mormon, not a church goer, not an advocate of any Christian faith. However, I have an interest in the sociology of religion and a particular interest in the LDS church. I find much about it paradoxical. I have observed that Mormons seem to want to pass as 'normal', just like me and you, only without the coffee, cigarettes and alcohol. This mainstreaming of the LDS population can probably be tied to the church officially denouncing the practice of polygamy in 1890 and Utah becoming a state. Ever since, the LDS church vigorously disavows the practice. Arguably the church's growth in the 20th century would not have happened without distancing itself from the practice of polygamy. Unfortunately, the structure of the religion as it was founded - anyone can receive a revelation from god - makes it rife for spin-off LDS sects embrace the original doctrine. Hence, you have several flavors of fundamentalist LDS groups living in remote insular communities where the men exploit, subjugate and sexually abuse woman and girls and justify it as their god sanctioned family values.
The author mentions on several occasions that the LDS church is unique in that it is relatively new and people, places and events are well documented. It is, although the Gold Plates from which Joseph Smith wrote the Book of Mormon are conveniently missing. BTW, he translated the BOM from these plates by looking a magic stone with his face in his hat. As it turns out, you don't need an airtight narrative to amass millions of followers.