I read about this story a while ago, and completely forgot about it, until Atheist Media posted this video:
Apparently, Mark Twain requested that his autobiography not be published until 100 years after his death, which has just past. Now that it has been a century, the University of California Berkeley will publish the first of what will be a three volume set of Twain's memoirs in November. As of now, there are 5,000 pages of his unedited thoughts in their vault.
I'm very excited to read this when it's finally published. Twain's humor and intellect is spot-on in all of his writing, and his critiques of religion, and specifically Christianity, are no exception. If you've not read his views on religion, I recommend Thoughts of God, from Fables of Man, which asks the question "Why would God create the fly?". I first read it in The Portable Atheist (which I also recommend very highly), and it's still one of my favorite essays. Twain has this uncanny ability to have you laughing while reading one paragraph, and in solemn contemplation during the next.
I would definitely be on the lookout for this in November when it's published.