The Allure of the Wilderness:
To MacCandless, wilderness has a specific taste to him, he believes that it is a place free of tensions and modern of society, this has been a reoccurring theme in many traveling books, which makes you understand that the allure of wilderness is overwhelming and enticing enough to cause a person to leave his people and live outside his society to feel what MacCandless first felt when he decided to go on a journey to find that free place.
However, that alluring felling will disappear as soon as you are there. MacCandless, for example, spent most of his time trying to find food and keep himself alive, so he did not even find time to cherish that wilderness!
So, as you see this is a central idea that runs through many travel books, which can help you understand the point of view from which the author is writing and also gives the story unity, influence and core.