WITH the recent discovery of the first half of the original draft of "The Adventures of Huckle berry Finn" and the imminent publication in The New Yorker of the "missing" cave chapter, there has been considerable talk of how Mark Twain wrote his masterpiece. Early versions and deleted chapters can, of course, cast revealing light on the mystery of how a classic came into being, especially when they show the author's aims and intentions at the time of composition. In the case of "Huckleberry Finn," however, there is another book, the rarely read travelogue "A Tramp Abroad," that tells us at least as much about Twain and the way he worked, and perhaps even his intentions, as the lost manuscript. Written and published in the years when he was blocked in his work on the novel that became the template for American literature, "A Tramp Abroad" is a book that, viewed from a certain angle, stands closer to "Huckleberry Finn" than any of his others. As it happens, it's also a book with which I have a special, and even a familial, affinity.

For me, the character who has always excited the most interest in "A Tramp Abroad" was not Samuel L. Clemens's garrulous, curmudgeonly traveler, his nom de plume, Mark Twain. Like most readers, I was certainly charmed by the world-famous American writer playing cat and mouse with his considerable fame and fortune and the by then venerable tradition of the journal of the European grand tour. Who wouldn't be? It was a tongue-in-cheek account written for those of his countrymen who had no intention of ever taking the tour themselves and who wished perhaps to feel virtuous about that decision, and its ideal audience was probably people who at bottom wanted someone knowledgeable to tell them that the journey wasn't really worth the not insignificant expense. It's a nonfiction genre that Clemens had earlier both parodied and extended with his immensely popular "Innocents Abroad" and its younger, less affluent, unknown (but no less crankily opinionated) narrator.

Generally, though, I myself preferred the man and his opinions as I read them in his Mississippi writings and in his great novels. This is not to say that Mark Twain was not grand good company. Quite the opposite. Unless I preferred traveling in silence, I would far rather make my way with him at my side than anyone else on this planet. One always, on closing a book narrated by Mark Twain, misses most his voice. With the possible exception of Huckleberry Finn's, it's the sly, sardonic, confiding and utterly seductive voice of Mark Twain, Clemens's other fine talker, that is the most characteristically American in our entire literature. We recognize it at once, joyfully -- dear, rumpled Uncle Mark, smelling of cigar smoke and after-dinner drink, speaking in funny accents and making up parodies and jokes and tall tales on the spot, but always switching to the somber side at just the right moment to restore to the conversation, or more properly, to the monologue, an appropriate dignity, so that we might by the end of it feel respected by this brilliant, witty man as intelligent, honest and impatient with hypocrisy.

Even so, in "A Tramp Abroad," which I first read nearly 30 years ago, it was not the narrator but his companion, his sidekick, who somehow engaged me in a continuing way. Mr. Twain was everywhere in the account precisely the man he appeared to be, but that Mr. Harris -- the anxious, often hapless, completely decent, if slightly humorless, fellow without profession or family, this American gentleman who acts in turn, and sometimes simultaneously, as the narrator's foil, fool, goad, guide and all-purpose straight man -- was a bit of a mystery. Strangely, until Chapter 10, three weeks into the tramp, Mr. Harris is but a shadow of Mr. Twain -- his unacknowledged second in the dueling society chapters, his silent dining companion in the Heidelberg chapters and his ignored roommate in the Schloss Hotel. Then, once he's properly acknowledged and more or less formally introduced, he's everywhere. He plays Butch Cassidy to Twain's Sundance, Sancho to his Quixote, Tonto to his Lone Ranger, Hardy to his Laurel. And, yes, Jim to his Huck.

But for all the humorous asides, exaggerations, tall tales and fantastical, obviously invented digressions of "A Tramp Abroad," the story itself is real. Is fact. And in spite of several omissions (Mrs. Clemens, their daughters, Susy and Clara, a companion for Mrs. Clemens and two servants were actually along), there is an implicit, underlying contract with the reader, providing the assurance that this journey was not invented out of whole cloth in Samuel Clemens's study back in Hartford, but that it actually took place, that it went approximately as described and that this fraternal pair of bourgeois gentilshommes is real.

And, indeed, just as Mark Twain, the Connecticut Yankee, is a literary stand-in for Sam Clemens, who grew up poor and marginalized on the banks of the Mississippi River, Mr. Harris, his roommate and hiking, rafting and carriage companion in Germany, Switzerland and Italy in the summer and fall of 1878, is a teasing disguise for the Rev. Joseph Twichell, pastor of the Asylum Hill Congregational Church of Hartford.

This is well known among Twain scholars, of course. It was not at all known to me, however, until about eight years ago, when my wife-to-be, surnamed Twichell, let it slip in conversation, not without a certain justifiable pride, that her great-grandfather was admired in the family for having been, among other things, Samuel Clemens's best friend and the minister who married him to his beloved Livy and presided over the burials of all the Clem enses, including that of the Great Man himself. Furthermore, Twichell had traveled to Europe with him, at Clemens's expense, and had been immortalized as Mr. Harris.

NATURALLY, as a fiction writer myself and a lifelong Twain devotee, I was delighted by this unexpected proximity to the man and his inner circle, even if only by dint of marriage and three generations too late. I remembered reading somewhere that Clemens took immense pleasure in staying up over good cigars and old brandy with his cronies, telling stories, singing songs and reciting his more scabrous works, even including the scatological "1601" and the heretical "Extracts From Adam's Diary." What, I wondered, did the good man of the cloth make of the reckless humor, the anger and the darkness of those late works? He must have been quite a tolerant and, to my mind, a wise pastor. Certainly he was less decorous, less conformist, less obtuse than poor Mr. Harris.

In the original edition of "A Tramp Abroad," Mr. Harris (to be honest, I can now think of him only as my great-grandfather-in-law) is pictured in numerous illustrations, some drawn by Twain himself in a deliberately (and comically) amateurish hand, as a tall, scrawny figure rather suggesting the schoolmaster Ichabod Crane, which happens nicely to match Twain's verbal portrait. My wife possesses a photograph of the pair, however. They stand alertly in the wind on the top deck of a steamship; the inscription written below, evidently by someone in the Twichell family, says, "Bermuda Bound!" The two look the way middle-aged men look when they're setting out to get in trouble -- a couple of runaways -- and the tall, athletically built Twichell looks quite as capable of outrageous behavior as his somewhat shorter friend.

From a written account less interested in literary effect than "A Tramp Abroad" and more concerned perhaps with literal truth, one also gets a telling picture. In a February 1876 issue of The Essex County (N.Y.) Republican, there is a charming description by the famous Adirondack guide Orson (Mountain) Phelps of a climb up Mount Colvin with three Hartford worthies, Charles Dudley Warner, Dr. Horace Bushnell and the Rev. Joseph Twichell: "Mr. Twichell strides off with more than an average pair of legs supporting the frame of a man in every respect," Phelps declares. "He is a thorough go-ahead man, vehement and impressive, sometimes amounting to rashness. He has a clear, powerful voice, which makes him an interesting public speaker."

HARDLY Mr. Harris. But he is the sort of man we would expect Sam Clemens to feel comfortable with -- a man as likely to long for escape from the confines of High Victorian Hartford respectability. It's an old male dodge, and not just in America (the earliest Western literary representation is probably the Homeric): the hunter-gatherer's dream of leaving the women and children behind and lighting out for the territory with his best male friend. And though the quest is usually portrayed as a young man's, for knowledge of the wider world, the fantasy is no less attractive for a middle-aged man, especially a man who, like Clemens and possibly like Twichell, had recently been running a little low on the inspiration and force that, up to now, had made him, in his own eyes and in the eyes of his community, successful. This painful conflict is treated too seldom these days with the sympathy it deserves.

So it should come as no surprise that Clemens's main literary obsession during these years was a fictional story narrated in the vernacular by a marginalized, homeless Southern boy with an escaped slave for his companion, the two of them drifting down the Mississippi River on a raft. And it should come as no surprise that he was finding himself unable to finish it.

Clemens, like many literary artists, characteristically put his material through three intimately linked stages before finally abandoning it and moving on. He was first an anecdotalist, then a journalist-memoirist and finally a fiction writer. One thinks of how Melville processed and transformed his experiences in the South Seas; or of Thoreau and Emerson, moving from private conversation and journal to sermon, pamphlet and athenaeum stump speech to personal essay or poem; or of Stephen Crane's three-staged transformation of his war experiences; or, in the 20th century, of Ernest Hemingway, and even of some contemporary writers, like Norman Mailer.

I observed this triple-jump sequence firsthand as a very young writer, when I came to know another American realist, Nelson Algren, then in his early 50's. I was eager to learn how a real writer worked, and in particular to see how a man who, except for the evidence of his books, seemed no more extraordinary than I. Algren liked to regale his listeners into the night (over cigarettes and whisky, instead of cigars and brandy) with his adventures in underground Chicago, Hanoi, Paris and New York. This was the early 1960's, and a few months later I would open a magazine or a new Algren book of nonfiction ("Notes From a Sea Diary: Hemingway All the Way," for instance, his own version of "A Tramp Abroad"), and there would be the same story, told more efficiently and powerfully and with a sharper point to the humor than I'd heard in conversation. Then, after a year or two had passed, I would read yet a third, the fictional, version (in his collection "The Neon Wilderness," for example), and by now all that remained of the original was the psychological truth and the mythic infrastructure, neither of which I had noticed before.

This is a useful procedure, to take one's fact-based fictional material out for a trial run, as it were, though one has to guard against draining the stuff of its dramatic meaning and structure or weakening its ties to one's deepest emotional life. It's a risky business; one has to husband one's small store of stories carefully, nurturing them often for years in the darkness of silence and solitude, before they can be understood enough to flower. But Sam Clemens, for whom silence and solitude seem to have been anathema, was especially skilled at the technique. It is an elaborate mode of public revision, actually -- inventing large segments of one's life, one's travels in particular (which are one of the more easily controlled, or willed, aspects of one's life), to provide material for late-night storytelling, even as the material for a nonfiction book already under contract is being organized, which book must later provide the basis for a work of literary fiction.

And in the case of "A Tramp Abroad," published in 1880, that latter work would turn out to be Twain's masterpiece, "The Adventures of Huckleberry Finn," published in 1884, after having been started and then several times abandoned in frustration and artistic unsureness, most recently just before Twain set off for Germany. "A Tramp Abroad," then, might well be read as an important revision of Twain's incomplete first draft of "Huckleberry Finn," a second draft, if you will, which in turn made possible the third and final draft, from which, as Hemingway noted, all American literature lies in direct descent.

Read this way, any number of episodes in "A Tramp Abroad" can be seen not simply as prefigurations of similar episodes in "Huckleberry Finn," but as trial runs; the idyllic raft trip on the Neckar (with Mr. Harris and crew aboard) and the raft's unexpected, violent destruction are obvious examples.

Finally, however, for me, the most intriguing parallel remains the sweetly symmetrical, interlocking pair of triangles through which Clemens-Twain-Huck is fraternally tied to Twichell-Harris-Jim. Clemens and Twichell, Twain and Harris, Huck and Jim were American males clearly blessed with the gift of friendship, of giving it and of receiving and holding on to it. And thus, perhaps because of my own somewhat recently acquired familial ties to Twichell, "A Tramp Abroad," of all Clemens's books, has turned out to be the most personally revealing of the literary genius himself, that very private man who lived his entire life ducking behind overlapping and often conflicting public personas. Through elaborate literary subterfuge, Samuel Clemens managed always in his greatest works to expose indirectly his impassioned heart. His solitary heart. But more so than anywhere else, in "A Tramp Abroad" he unveils as well his abiding commitment to that most democratic of virtues, fraternity, which, surely, all through his turbulent middle and late years soothed and calmed his rage.

A longer version of this essay will appear as the introduction to the facsimile edition of "A Tramp Abroad," which Oxford University Press is publishing next year.

Photo: Mark Twain at sea in 1907 with the Rev. Joseph Twichell of Hartford, a close friend and model for the character Harris in "A Tramp Abroad."