Mark Twain’s autobiography is very disconnected. The chapters have almost no correlation to each other and have no order, and the book isn’t unified. There are certain reasons this disjointedness occurred.
First, Mark Twain’s autobiography was written in an interesting way. He dictated every chapter separately, and the subject, while most of the time was generally going in the direction of his personal history, was said to be whatever interested Twain at the moment, resulting in tangents.
Second, Twain requested that his narrative not be edited. It was to remain as is. However, editing is an important tool for writing, and the biography would have been more complete had it been used.
Third, Twain’s mental health was steadily decreasing throughout the time he narrated his biography. He had always been eccentric, but the deaths of various close family members that occurred during the writing of the biography degraded his mental state. He was nigh mad with sorrow by the last chapter, when the last close family member he had, his daughter, died. The last chapter, and by extension, the book, ended abruptly after the mention of her death.
Out of all three of those reasons, however, the fact that Twain requested that the autobiography not be edited was probably the most prevalent of the reasons. If it had been edited, that would have fixed the majority of the issues people seem to have with it. However, I think that leaving it as is was the right decision. It adds character, even if it was a bit disjointed at times.