Under the Banner of Heaven
Here's one that people all around me seem to be reading: Under the Banner of Heaven: A Story of Violent Faith, by Jon Krakauer. It's an in-depth look at the world of Fundamentalist Latter Day Saints (FLDS). These are folks who have returned to the original teachings of Mormon Founder Joseph Smith, especially with regards to "The Principle" of plural marriage (polygamy). They accuse the "official" Mormon church/cult of having abandoned the genuine faith of Smith.
Krakauer himself is not a Christian, and sometimes he makes blanket statements against religion in general that rub me wrong, but he has done his homework thoroughly on the origin of the Mormon church/cult and current FLDS groups, which is beyond helpful. I couldn't possibly get into a comprehensive review of this book without spoiling it for you, so I strongly recommend you pick up a copy for yourself. I promise you will be floored when you read about the communities and practices of the FLDS groups Krakauer examines.