Conversation so far:
What is Jon Krakauer's Under the Banner of Heaven?
Jon Krakauer's third non-fiction bestseller.
What is Jon Krakauer's book, Under the Banner of Heaven about?
The book examines extremes of religious belief, specifically fundamentalist offshoots of Mormonism.
What year was Jon Krakauer's Under the Banner of Heaven written?
In 2003.

Question: Why did he write this book?
Self-contained rewrite: Why did Jon Krakauer write Under the Banner of Heaven?