Krakauer says he is “not an impartial biographer.” What does this mean and why does he share this with the reader in the beginning of the book?

John Krakauer saw many similarities between himself and Chris. By including notes about his own life, Krakauer is able to draw similarities between his own life, and his motivations for writing the book, with Chris's life. The narrative hence becomes a personal recognition of the Chris's personality type, the ever exploring searching soul. Krakauer doesn't claim to be objective. He wants to present Chris in his own interpretation.