IN COUNTRY. By Bobbie Ann Mason. Harper & Row. 247 pp. $15.95.

ON THE STRENGTH of her previous book, Shiloh and Other Stories, Bobbie Ann Mason became a minor literary celebrity, a phenomenon for which she is not to be held responsible but which complicates one's reading of In Country, her first novel. Like Jayne Anne Phillips and Carolyn Chute, Mason writes about life in rural America, a place now much in fashion among urban sophisticates who are attracted by what they fancy to be the farm's earthiness and quaintness. This readership has found in the work of these writers precisely what it wants, and as a result has showered them with honors and praise; for Shiloh, Mason won an Ernest Hemingway Foundation Award and was a short-list nominee for three major literary prizes.

It's easy enough to see what these sophisticated readers like about Mason's fiction. It is set in Kentucky, a state that for most of them probably exists entirely in the imagination, and it deals with small-town folk who are trying to cope with the homogenization of America, a question that always arouses indignation among the very people who have contributed so much to it. Much of her dialogue is in the Maw-and-Paw style favored by Sally Fields and other portrayers of courageous country people. Many of her strongest characters are women, which provides a convenient bond between the rural housewife and the urban feminist. She is opposed to war in general and the Vietnam War in particular.

All of which is perfectly all right; writers are entitled to choose their subjects, and readers are entitled to praise whatever they wish in their books. But you have to cut through a thick cloud of extraneous material to read Mason's work with objectivity, and when you do so you find considerably less than you had hoped for. The stories in Shiloh are amiable and appealing, but they are all pretty much alike and eventually they dissolve into a blur. They are more accomplished and interesting, though, than In Country, the good intentions of which are defeated by thematic predictability, unenticing characters and endless, pointless chatter.

There is much similarity between In Country and Jayne Anne Phillips' Machine Dreams, though Mason does not go in for prettified prose or overwrought symbolism. Both novels are about country people whose lives were permanently altered by Vietnam, both have much to do with what Mason and Phillips believe to be the male propensity for armed combat, and both center on young women who gradually awaken to the war's deeper meanings. Both, in other words, address unsophisticated rural life with sophisticated sensibilities; Mason and Phillips write about people called Maw and Paw, but their view of these people is neither rustic nor provincial.

The young woman in Mason's novel is Samantha Hughes. She is called Sam and is 17 years old, soon to be 18. She lives in a Kentucky town called Hopewell with her 35- year-old uncle, Emmett, a Vietnam veteran who may or may not be a victim of Agent Orange. Her father, Dwayne, was killed in Vietnam shortly before her birth; now her mother, Irene, lives in Lexington with her prosperous new husband and infant daughter. Sam is about to head off for college, but before she does she feels a need to come to terms with Vietnam, a conflict of which she is an innocent victim.

THIS INVOLVES her in long if somewhat one-sided conversations with Emmett, who uncooperatively conforms to the clich,e that veterans don't like to talk up their war experiences with the wimminfolks, and then with Tom, another veteran on whom she develops a strong crush. This leads her to attend a dance intended to draw local attention to what the vets did for their country, but even in patriotic Hopewell, not many people appear because not many people care -- a theme that Mason bleeds for all it's worth and then some. As Tom says: "Sam, you might as well just stop asking questions about the war. Nobody gives a shit. They've got it twisted around in their heads what it was about, so they can live with it and not have to think about it. The thing is, they never spit on us here. They treated the vets O.K., because the anti-war feeling never got stirred up good around here. But that means they've got a notion in their heads of who we are, and that image just don't fit all of us. Around here, nobody wants to rock the boat."

But Sam, doughty little Sam, insists on asking questions. Eventually her determination leads her to letters her father wrote from Vietnam and a diary he kept there, a diary that opens her eyes to the brutal realities of war. In hopes of experiencing those realities herself, she undertakes a melodramatic expedition "in country," to a pond "so dangerous the Boy Scouts wouldn't even camp out there, . . . the last place in western Kentucky where a person could really face the wild." Then, in the novel's climax, she goes on a hegira to the Vietnam Veterans Memorial in Washington with Emmett and her paternal grandmother, who incredibly enough is called Mamaw.

All of this is meant to be painful and illuminating, but any power it might possess is eviscerated by its dreary familiarity. That many veterans of Vietnam have been put through hell is indisputable, and that the honor they have lately received is long overdue is equally so, but these points have been made many times before and nothing that Mason says adds anything to our understanding of them. Further, she has failed to transform these essentially political questions into the stuff of fiction; none of her characters comes to life, the novel's structure is awkward and its narrative herky-jerky, her prose wavers uncertainly between adult and teen- aged voices. Her heart may be in the right place, but that isn't enough to bring In Country to life.