The dominant forms of appeal in Christopher McCandless' letter as published in Into the Wild by Jon Krakauer are ethos and pathos.
Although his letter was originally designed to convince a specific person (Ronald Franz) to take up a more adventurous lifestyle, his use of ethos and pathos also helps make the letter convincing to a more general audience.
Through ethos, McCandless attempts to make himself appear trustworthy, skilled, and knowledgeable in Ron’s eyes. First, McCandless relies on his personal experience to convince Ron that he is trustworthy. For example, the beginning of the letter is almost exclusively focused on McCandless and his experience in South Dakota: “I have been working up here in Carthage, South Dakota for nearly two weeks now. I arrived up here three days after we parted in Grand Junction, Colorado” (p. 208, ll. 1-3). As McCandless portrays himself as a man who works hard, it is more likely that Ron will take him seriously.