The unpublished "A Family Sketch" was a 64-page, handwritten manuscript that Twain wrote around 1896 or 1897 for Olivia "Susy" Clemens, who inspired some of his stories and even wrote her own biography of her father. The document also reminisced about his own childhood and was described as the missing chapter of his autobiography.

"What initially began as a tribute to his late -- and undisputed favorite -- daughter Susy thus devolved into a narrative that encompasses the whole of this family and friends as well as glimpses of incidents of his own childhood," auctioneer Sotheby's said in a statement.

Scholars are divided as to why Twain wanted the first-hand account of his life kept under wraps for so long. 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.

One thing's for sure: by delaying publication, the author, who was fond of his celebrity status, has ensured that he'll be gossiped about during the 21st century. A section of the memoir will detail his little-known but scandalous relationship with Isabel Van Kleek Lyon, who became his secretary after the death of his wife Olivia in 1904. Twain was so close to Lyon that she once bought him an electric vibrating sex toy. But she was abruptly sacked in 1909, after the author claimed she had "hypnotised" him into giving her power of attorney over his estate.

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.

"You had better shove this in the stove," Mark Twain wrote to his brother in 1865, "for I don't want any absurd 'literary remains' and 'unpublished letters of Mark Twain' published after I am planted." Despite this, a collection of previously unpublished stories and essays by the great American writer are due out this April, almost 99 years after his death.

The title of the collection, Who is Mark Twain? is a reference to Twain's essay Frank Fuller and My First New York Lecture, included in the book. In the essay, Twain relates how – anxious that no one would attend – he plastered New York with advertisements to promote his talk. He later observed two men looking at the ads. One asked, "Who is Mark Twain?", to which the other responded: "God knows – I don't."

Another essay, Jane Austen, sees Twain – born Samuel Clemens in 1835 – ask if Austen's goal is to "make the reader detest her people up to the middle of the book and like them in the rest of the chapters", while the previously unpublished short story The Undertaker's Tale is a tongue-in-cheek piece about the funeral industry.

HarperStudio, the publisher of the 24-piece collection put together by Robert Hirst, general editor of the Mark Twain Project at the University of California, said that Twain left behind the largest collection of personal papers created by any 19th-century American author. "The pieces themselves are wonderfully, hilariously contemporary, and deserve as wide an audience as possible," said publisher Bob Miller.