Their ill-fated relationship will be recounted in full in a 400-page addendum, which Twain wrote during the last year of his life.
It provides a remarkable account of how the dying novelist's final months were overshadowed by personal upheavals.
However, Robert Hirst, who is leading the team at Berkeley editing the complete text, says that more than half of it has still never appeared in print.
Only academics, biographers, and members of the public prepared to travel to the university's Bancroft research library have previously been able to read it in full.
"There are so many biographies of Twain, and many of them have used bits and pieces of the autobiography," Dr Hirst said.
"But biographers pick and choose what bits to quote.
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Another potential motivation for leaving the book to be posthumously published concerns Twain's legacy as a Great American."Most people think Mark Twain was a sort of genteel Victorian.Well, in this document he calls her a slut and says she tried to seduce him.Some believe it was because he wanted to talk freely about issues such as religion and politics.
Others argue that the time lag prevented him from having to worry about offending friends.
The autobiography will perhaps show that it wasn't such a happy time.