Non-fiction of such excellence is only possible through a considerable amount of hard work and effort. The research that it takes to investigate such a story occupies many hours and involves much diligence on the author's part. You might like to consider the "Acknowledgments" section of the book at the very end, following the final chapter, where Jon Krakauer notes down a series of people that shows the extent of his research and the involvement of others in his work. Note how the first paragraph of this section of the novel makes the debt that Krakauer owes to the family of Chris McCandless evident:
Writing this book would have been impossible without considerable assistance from the McCandless family. I am deeply indebted to Walt McCandless, Billie McCandless, Carine McCandless, Sam McCandless, and Shelly McCandless Garcia. They gave me full access to Chris's papers, letters, and photographs and talked with me at great length.
This of course shows one of the major sources of research for this work, but at the same time we can also infer that considerable time was spent interviewing other characters that appear in this book and also researching the flora and fauna and literature that is so important for this tale.