Chris survived 112 days in the Denali area. But after eating some moldy wild potato seeds, he became sick and died of stavation. Krakauer surmises that McCandless died from the mold (Rhizoctonia leguminicola) which "produces a potent alkaloid called swainsonine" which also is known to kill livestock who eat damp forage.
The book also digresses a few times and devotes chapters to other adventures. One chapter discusses other men who have tried face the land of Alaska alone. Some failed while others succeded. He also devotes a chapter to Everett Ruess, who similiar to McCandless, lived off the land (in Utah and Arizona), but who disappeared and nothing is known of his death (if indeed he died). The author also sets aside one chapter to explain his fight with the Devil's Thumb ... a legendary mountain in Alaska.
The one thing that bothered me about the book was how the author took his sweet time in telling the story. He digressed quite a few times to explain the lives of other adventurers like McCandless. He also injected quite a bit of personal history into the book too. But once I realized he was going to do this, I just sat back and enjoyed the ride.
What amazes me is that there are a lot of people who wander. There seems to be a whole culture around hitch hiking, living off the land and having little or no possessions. If ever there is a nuclear holocaust and these people are the only ones who survive, our new civilization will be quite different from the one we live in today.