what exposition is provided by Huck, which he claims is the prequel to this story

The novel begins with Huck explaining that we have heard of him from another book. Most of us know that he is alluding to "The Adventures of Tom Sawyer". Huck also fills us in that it was written by "Mark Twain". Huck adds that if we have not heard of him "it ain't no matter". Huck vouches for twain's credibility while others, like the female characters, tell lies. Huck then goes on to explain what happened with the reward money ($6000 a piece that Judge Thatcher invests). He talks about how the Widow Douglas has failed to reform him. The narration carries Huck's literary voice. He is determined to live a life free of labels and conformity. He is earnest and good natured.