The New York Times has a report on a "rediscovered Mark Twain fairy tale" -
But decades later, the scholar John Bird was searching the Twain archives at the University of California, Berkeley, when he came across the notes for the story, which Twain titled “Oleomargarine.” Mr. Bird was astonished to find a richly imagined fable, in Twain’s inimitable voice. He and other scholars believe it may be the only written remnant of a children’s fairy tale from Twain, though he told his daughters stories constantly...I think it's only fair to point out that "The manuscript was just 16 handwritten pages long, and unfinished" and that Mr. Bird "wrote his own version, which closely followed Twain’s blueprint and incorporated his language," using "Twain’s spare ur-text," and that the Steads subsequently created a 152-page "wonderful story inspired by Twain’s unfinished manuscript, which makes any Twain purist uneasy."
After consulting a few other scholars, Mr. Bird brought the text to the attention of the Mark Twain House and Museum in Hartford, which sold it to Doubleday Books for Young Readers. This fall, Doubleday will release “The Purloining of Prince Oleomargarine,” an expanded version of the story that was fleshed out and reimagined by the children’s book author-and-illustrator team of Philip and Erin Stead.
This is interesting, but I hope it isn't over-hyped. Personally I would rather read the original manuscript and imagine the rest for myself. I hope they will incorporate the unmodified original into the new publication.
Here's the Wikipedia entry on the history of oleomargarine. Twain could have encountered it in the mid-1860s when he was in California gold rush territory.