NEW YORK, FEB. 13 -- The handwritten manuscript for half of Mark Twain's "The Adventures of Huckleberry Finn," one of the touchstones of the American imagination, has been discovered in a California attic after being lost for more than a century.

The 665 pages, heavily annotated in pen and ink, comprise the first half of the novel, according to Sotheby's, the auction house, which announced the discovery today. The manuscript for the second half has been at the Buffalo and Erie County (N.Y.) Public Library since the 1880s.

"I'm stupefied," said Kenneth Sanderson, an editor of the Mark Twain Project at the University of California at Berkeley, which is publishing scholarly editions of all of Twain's work. " 'Huckleberry Finn' is the nearest thing we have to the Great American Novel in this country. We had all given up on the manuscript long ago. Even the author thought it had been destroyed."

Perhaps the most American of books, "The Adventures of Huckleberry Finn" was first published in England in 1884. Not until the next year did it appear in the United States, where the first critical reactions ranged from silence to abject derision.

The book, mining many of the classic themes of American literature and life, eventually hit its mark. The story of a rebellious boy's coming of age on a Mississippi River raft trip, it presaged such hallmark characters of American fiction as J.D. Salinger's Holden Caulfield in "Catcher in the Rye" and Saul Bellow's Augie March in "The Adventures of Augie March."

Nobody drew on the quirky, desperate and lovable character of Huck more than Ernest Hemingway, who was never disputed when he uttered the famous line, "All of modern American literature comes from one book by Mark Twain called 'Huckleberry Finn.' "

But the novel was far more than an epic about a boy growing up in a violent society. It also was about the American frontier and the nation's tortured relationship with religion. In its painful look at the treatment of Huck's raft companion, a slave named Jim, it provides one of the most telling accounts of American race relations ever published.

"I think it will be some time before the true significance of this discovery can fully be known," said David N. Redden, director of Sotheby's books and manuscripts division. "But I think I can say without any doubt that it is the most valuable discovery of American literature we have ever seen."

Twain, a pen name for Samuel Langhorne Clemens, started the book in 1876 but put it away after a couple of years because, as he wrote at the time, "the well ran dry." Because he stopped and started it many times over the years, this heavily rewritten manuscript has special significance for scholars.

Written neatly on small lined note paper, the document appears to vary widely from the original published text, according to Sotheby's officials, and includes at least one large section not published in the first edition.

The manuscript is of particular interest because the story is written in dialect, making it likelier that printers of the time would introduce spelling and punctuation errors.

Twain was infuriated because printers commonly made changes. On one occasion, he said his publisher had written "that the printer's proofreader was improving my punctuation for me, and I telegraphed orders to have him shot without giving him time to pray."

The second half of the manuscript has been in Buffalo since Twain sent it there at the request of James Fraser Gluck, a lawyer and local civic leader. Gluck had asked for the original manuscript of Twain's "Life on the Mississippi," but since that was unavailable, Twain sent "Huckleberry Finn."

At the time, Twain wrote Gluck that the first half of the manuscript had been destroyed accidentally at the printer. He rediscovered it two years later and sent it to Gluck, who scholars thought had discarded it mistakenly.

Gluck's granddaughter found the manuscript in an old trunk last fall and later sent it to Sotheby's for authentication. Officials there said today that it almost certainly would be sent to Buffalo.

"Persons attempting to find a motive in this narrative will be prosecuted," Twain wrote in what has become perhaps the most famous prefatory note in American literature. "Persons attempting to find a moral in it will be banished; persons attempting to find a plot will be shot."

If that note had been enforced, countless thousands surely would have been taken to court, exiled or executed by now. The book, one of the most popular ever published with more than 30 current editions, has been interpreted and written about by almost everyone who is part of the nation's literary history.

To critic Lionel Trilling, the book was foremost an examination of the relationship between the individual and society. Huck's harrowing flight downriver was merely a painful, picaresque attempt to figure out where he fit.

To critic Leslie Fiedler, the tense, idiosyncratic relationship between Huck and Jim was a stark examination of a homosexual relationship.

But to most readers, "The Adventures of Huckleberry Finn" has simply been solid, captivating, chilling fun. In this story of a boy who never doubts that he did wrong to help a slave run away, of a sweet innocent who sees more than a dozen dead bodies in his gruesome journey, Huck never takes his finger off the central contradictions of American life.

"I felt good and all washed clean of sin for the first time I had ever felt," he said after writing Jim's owner to tell her where her escaped slave could be found. "And I knowed I could pray now. But I didn't do it straight off, but laid the paper down and set there thinking -- thinking how good it was all this happened so, and how near I come to being lost and going to hell.

"And went on thinking . . . and I see Jim before me all the time: in the day and in the night-time, sometimes moonlight, sometimes storms, and we a-floating along, talking and singing and laughing. But somehow I couldn't seem to strike no places to harden me against him, but only the other kind."