Jim Harrison is a pretty average-size guy, and yet you need to spend only a few hours in his presence to realize just how gargantuan he really is.

First, of course, there are the 25-plus novels, collections of stories, poems and essays he has written during his 40-odd-year career. Then there's the legend of appetites: the drinking, the smoking, the eating, the carousing for which he and such pals as Jack Nicholson are famous.

But high literary allusions cheek by jowl with silly, sometimes childish wordplay? Who knew? In my marathon daylong-plus interview with Jim Harrison in Grand Rapids, Mich.—where he teaches at Grand Valley State University five days a year—he told the class of undergraduates, for instance, that he counted among his influences Rimbaud "the French poet, not the character played by Sylvester Stallone." While backseat-driving his assistant, Joyce, around town, looking for a liquor store to purchase the vodka he mostly drinks now that his doctor told him his beloved wine is bad for his diabetes, he spouts a line of familiar poetry. "Who wrote that?" I ask him. "Yeats," he says. "Billy." Maybe because he gets the reaction he wants—Joyce nearly drives off the road, we're laughing so hard—a few minutes later he regales us with some lines from Keats, Johnny.

(Never mind that some of the jokes were the you-had-to-be-there kind, but only if "there" was the seventh-grade locker room: about a pink-haired student on the campus, he allowed as how he wouldn't touch her with... well, you know the rest.)

But here's what's irresistible about Harrison's humor: he slays himself. He's hugely amused at how amusing he is. When he's having fun, he laughs, he barks, he coughs, he wheezes—he has a cold, he says, but he also smokes a couple of packs of American Spirits a day—and then he fixes you with a cockeyed stare that at first is unsettling, thanks to his wandering, half-blind left eye. But it's not long before you find the whole act charming; the total effect is of a slightly mad, aging elf, a Rabelaisian observer who's had plenty to drink.

The occasion for this interview is Harrison's new novel, Returning to Earth, which his longtime publisher, Grove Atlantic, will release in January. It's a wonderful, rich and moving book about Donald, a middle-aged half-Chippewa husband and father dying of Lou Gehrig's disease. Told in several voices—Donald; his wife, Cynthia; her brother, David; and Donald's daughter, Clare—it explores the Harrisonian themes familiar to readers of, say, Dalva or Julip or his 1998 effort, The Road Home: ethnicity, love, sex, family and, of course, the natural world. But every time we get on the subject of the book, we get right off it, somehow. Or do we? A conversation with Jim Harrison cannot, apparently, ever be about one thing: When I ask him why he, the Michigan-born son of white parents, writes so often about Native Americans, and whether there's anything autobiographical in his mixed-race characters, he waves the question away. "Nothing in the world causes more problems than concepts of ethnic virtue," he says in his flat, north-midwestern twang. "It's irrelevant. In northern Michigan, in the UP [Upper Peninsula] I would say over half are mixed-blood people just because people are sexual and they just merged together. It's that melting-pot crap we learned in high school." Then he stops and smiles, coming back around to the question after all. "I'm just like Donald: ranting about the melting pot."

In fact, anything you want to know about Jim Harrison—or at least about Jim Harrison's version of Jim Harrison—you can read in his 2002 memoir, appropriately titled Off to the Side. Here are the stories that shaped him, that come up again and again in his books and his conversation: the car crash that killed his father and 19-year-old sister; the accident that nearly blinded him; the miserable, if profitable years he spent as a screenwriter in Hollywood ("If you're an actual novelist, you get treated a lot better than [just] a screenwriter.... They cheat you, but you get cheated at a [high financial] level you find inconceivable," he says now.); those famous appetites; and his passionate, continuous obsession with the natural world. His conversation, like his books, is laced with references to the wild, to wildlife, to animals. He tells me he prefers to stay in a particular hotel in Paris because he can "sit in the bath and see birds." He tells the students today that the inspiration for the novella they're studying, The Beast God Forgot to Invent, was a dream he'd had about eating a bear.

"We pretend that the brain is binary, like a computer," he tells the students in answer to a question about where his ideas come from. "But it's not. It's completely holographic."

In other words, it's "all over the place," another good way to describe Harrison himself. Here, for example, is a guy who says he hates doing interviews because he is constantly asked the same questions—the best ones, he says, are in France, where he is a bona fide bestselling author and where interviewers are likely to ask such sophisticated questions as "How much was your work affected by that of [Canadian novelist] Robertson Jeffries?"—but answers any question you ask him by invoking memory, dreams, books, gossip. He can talk as animatedly about such pals as fellow author and Montana resident Tom McGuane—with whom he has been exchanging weekly letters for 40 years—and Nicholson, whom he met through the movie Wolf—as he does about his late, beloved brother. (Two years older, John Harrison was a librarian at Yale and the only person to whom Harrison sent the only copy of the manuscript of his first novel, Wolf. Problem was, it landed in New Haven at the start of a mail strike, so the elder Harrison—"a very imposing, powerful, human being," according to his brother—had to talk his way into the post office to find it.) He doesn't have much patience for discussing his writing habits (every day, longhand; assistant Joyce types up and sends), but he's quick to praise his longtime Grove editor Amy Hundley and to tell me the story—twice—about his relationship with Grove head Morgan Entrekin. It goes like this: Legendary editor Seymour "Sam" Lawrence, who published Harrison's first commercial success, Legends of the Fall, was dying. Lawrence's protégé, Entrekin, was just in the process of founding Atlantic Monthly Press. Everybody was worried, he said, because Morgan had no backlist. (Entrekin soon remedied that by buying Grove Press.) Still, Lawrence looked up from his deathbed—and here is where Harrison lowers his eyes and waves his arms theatrically—and croaks, "Stay with Morgan. Staaaaaay with Morgan." And so he has—for over 20 years. "He doesn't have as much money as some publishers," Harrison says, "but at my advanced age [he turns 69 in December], that can't be the key thing."

The "key thing," of course, is to keep on writing—and Harrison says that a few days after finishing Returning to Earth, he started a new, comic novel about academia. He also allows that it is important to take oneself seriously but not too—too many authors, he says, are fundamentally humorless about themselves. As he writes me in an e-mail a few days after he's left Michigan for his beloved Montana. "An afterthought while walking the dogs in the mountain valley with an actual golden eagle soaring overhead.... Like those in the news media, writers tend to think of themselves as terribly important and serious because they deal with serious and important issues: love, death, war, etc. For both, in historical terms, this is an imperfect self image."

Imperfect "off-brand geezer" (his words) though he may be, Harrison has struck his own kind of balance. "I won't talk or deal with a young writer unless I sense he has utterly given his life over to it," Harrison says. "It's a waste of my time. If they don't feel 'called'—why in God's name would you do this?" On the other hand, he says with a laugh, "I hang in there because I don't know what the fuck else to do."