Twain is writing in a style that you could call "vernacular" if you were feeling fancy, and "ordinary speech" if you were feeling, well, ordinary. (Vernacular specifically refers to language as it's spoken in particular regions—so, the way that Valley Girl speech is different from the way they talk on Jersey Shore. If that's even language.)
The problem is, Twain was writing in the nineteenth century, and ordinary speech has changed a lot since then. So, we get lines like "It was an awful sight of money when it was piled up" (1.2), or "he set down on the ground betwixt me and Tom" (2.5). These sentences aren't hard, but they're also not quite as easy as reading status updates on Facebook.
One more thing: don't take any grammar lessons from Huck. Trust us.