The word "mainly" shows that the story of Huck Finn is more fiction than fact. He repeats this word many times when he is talking about the story being one of truth. This intensifier introduces the idea of the story being a bit exaggerated and fictional. The other words add to the characterization of Huck. He uses ain't simply to mean isn't. When he says "that ain't no matter" he means that it is no big deal. Without is used to mean exactly that and is not meant as an opposite.
Thoreau has a possitive attitude toward the outdoors and believe that all people should return to nature as salvation for them. He believes that cities are bad for us.
