Krakauer was born in Brookline, Massachusetts, as the third of five children of Carol Ann (nee Jones) and Lewis Joseph Krakauer. His father was Jewish and his mother was a Unitarian, of Scandinavian descent. He was raised in Corvallis, Oregon, from the age of two. His father introduced the young Krakauer to mountaineering at the age of eight.

In 2003, Under the Banner of Heaven became Krakauer's third non-fiction bestseller. The book examines extremes of religious belief, specifically fundamentalist offshoots of Mormonism. Krakauer looks at the practice of polygamy in these offshoots and scrutinizes it in the context of the Latter Day Saints religion throughout history. Much of the focus of the book is on the Lafferty brothers, who murdered in the name of their fundamentalist faith.  In 2006, Tom Elliott and Pawel Gula produced a documentary inspired by the book, Damned to Heaven.  Robert Millet, Professor of Religious Understanding at Brigham Young University, an LDS institution, reviewed the book and described it as confusing, poorly organized, misleading, erroneous, prejudicial and insulting. Mike Otterson, Director of Media Relations for The Church of Jesus Christ of Latter-day Saints (LDS), told the Associated Press, "This book is not history, and Krakauer is no historian. He is a storyteller who cuts corners to make the story sound good. His basic thesis appears to be that people who are religious are irrational, and that irrational people do strange things."  In response, Krakauer criticized the LDS Church hierarchy, citing the opinion of D. Michael Quinn, a historian who was excommunicated in 1993, who wrote that "The tragic reality is that there have been occasions when Church leaders, teachers, and writers have not told the truth they knew about difficulties of the Mormon past, but have offered to the Saints instead a mixture of platitudes, half-truths, omissions, and plausible denials". Krakauer wrote, "I happen to share Dr. Quinn's perspective".

Answer the following question by taking a quote from the article: What was the focus of the book?
The book examines extremes of religious belief, specifically fundamentalist offshoots of Mormonism.