Many parts of the narrative are based on Krakauer's research into McCandless and his life.
Unwilling to let McCandless go, I spent more than a year retracing the convoluted path that led to his death in the Alaska taiga, chasing down details of his peregrinations with an interest that bordered on obsession.
Krakauer explains in the book's opening author's note that he spent a great deal of time trying to retrace McCandless's journey. Frequently, Krakauer incorporates into his narrative letters that McCandless wrote to various people during his travels. Krakauer also recounts many interviews he conducted with people that knew McCandless as more than a passing vagrant. All of those are examples of Krakauer's research.
Chapters 8 and 9 are the two most glaring pieces of evidence that Krakauer's narrative is based on research. That's because those chapters focus on men that came years before McCandless and lived or died in similar fashions to McCandless. For example, the last piece of first hand evidence regarding the disappearance of Everett Ruess is dated 1934. That's well before either McCandless or Krakauer was alive. The only way for Krakauer to include so much detail regarding that man was to do extensive research.