RIDE WITH ME, MARIAH MONTANA By Ivan Doig Atheneum. 324 pp. $18.95

IVAN DOIG is a writer whose work makes readers recall why they love to read, reminds writers why they ever wanted to write in the first place. Doig's characters befriend you. His landscapes bowl you over. His novels lay whole worlds at your feet and invite you to make them your own.

Ride With Me, Mariah Montana is the final (alas) installment of a trilogy begun with English Creek (1984) and followed by Dancing at the Rascal Fair (1987). Readers of the first two volumes will likely find this ride worth the wait, but those who missed the earlier books shouldn't have any trouble keeping up with Mariah, either. This novel stands on its own.

And it stands, as well, apart. Ride With Me, Mariah Montana continues Jick McCaskill's account of his Scottish family, of their fruitful and spirited inhabitation of Montana's Two Medicine country. But it is 1989 now. McCaskills have lived within sight of Phantom Woman Mountain for about a century, Jick for 65 years, and the state of Montana is getting ready to make the most of its 100th birthday. English Creek, lined with willows, still curves down out of the mountains, but it lingers on the edges of a changed world, a world afflicted by "the longest epidemic of all, loss." You can still watch buffalo roam in Montana, provided you're willing to track them "from a motorhome the size of a small boxcar." You can still spot a bull elk, but it's mounted over a pasty-faced city editor's desk. Grizzly bears die of relocation. Entertainment is provided by "Montana's homegrown C-and-W group, The Roadkill Angels," and by the latest in t-shirt slogans -- "Keep Montana Green, Shoot a Developer."

No wonder, then, Jick McCaskill has grown a little irritable, loss around him everywhere he looks. His beloved wife has recently died, and Jick is starting to suspect the hard business of ranching is getting to be too much for him. A nearby corporate ranch is licking its chops over the McCaskill land. Jick's hopes of keeping the ranch in the family were shot down three years ago with the divorce of his daughter Mariah and her husband, Riley Wright.

Feeling the reins of present and future slip from his hands, Jick is, more than ever, haunted by the past. ("Doesn't time know any statute of limitations, for Christ's sake?") And he is still trying to reassemble a rather crochety family history:

"Every family is a riddle . . . People on the outside can only glimpse enough to make them wonder what in the name of Jesus H. Christ is going on in there . . . while those inside the family have times, sometimes lifetimes, of being baffled with one another . . . parent and child . . . eyeing each other like foreign species. Knots in the bloodline. The oldest story there is, and ever the freshest."

Preoccupied, perhaps, with a lifetime of bafflement, Jick gets himself into a real predicament: Before we quite know what's got into him, he has signed on for a crisscross centennial tour of the sorry state of Montana with his daughter and her ex-husband. Riley and Mariah, still more than half in love with each other, work for a Missoula newspaper, he as a columnist ("Wright Angles"), she as a photographer. Their assignment, hard-won and loosely defined, is to piece together a portrait of Montana in all the rich contrariness of its hundredth year.

Jick's Winnebago makes for mighty close quarters, as Riley and Mariah follow their noses. Such bickering, such chemistry . . . it's a scenario for Tracy and Hepburn. And there is Jick -- "letting myself get just exactly where I knew not to get, between the pair of them" -- digressing a blue streak and sputtering like Walter Brennan.

It is the sputter of Jick McCaskill's voice -- cranky, confused, honest, stubborn and lovelorn -- that orchestrates the journey, reconciles past and present and makes the whole novel sing. Ride With Me, Mariah Montana is, like its narrator, not without flaws. Old men tend to ramble, and this particular would-be codger has a weakness for one-liners that occasionally gets the better of him. Jick is so opinionated, especially where his daughter and her ex-husband are concerned, that Mariah and Riley remain a bit dim for a reader, overshadowed by the narrator's anxiety, aggravation and love. And these poetic columns of Riley's in a small-town paper? Such gloriously overshot writing hasn't been seen since James Agee left Fortune magazine.

But Ivan Doig is a writer committed to plenty, and maybe plenitude always involves a touch of excess. Ride With Me, Mariah Montana is an extravagant celebration and, above all else, a love story. It is filled with devotion and passion for its locale, its people and their history, and smitten with the very language from which it is fashioned. It is, as journeys tend to be, a little episodic. But some of its turns and destinations are wonderfully surprising. And what a ride. I wouldn't have missed it for the world. Susan Dodd's most recent book is "Hell-Bent Men and Their Cities."