The Bancroft Library at UC Berkeley houses the manuscript(s) for this work. They will release the first of three planned volumes in November.
As The Independent says,
The creator of Tom Sawyer, Huckleberry Finn and some of the most frequently misquoted catchphrases in the English language left behind 5,000 unedited pages of memoirs when he died in 1910, together with handwritten notes saying that he did not want them to hit bookshops for at least a century.
Well, 100 years have come and gone. What will it reveal? Did he require the delay so he could speak freely on his beliefs concerning religion and politics or was it to protect important relationships? Or was it only to preserve his legacy? Are these mutually exclusive? About half of this autobiography has already been published here and there. Twain himself allowed the publication of some of it in his lifetime.
Check out The Independent’s article for more details and some reflections on the contents of the autobiography by a few who have seen it.