Until Gwen Moffat is within at least pondering distance of the Donner Pass, this is one of those ideas that, without going seriously wrong, doesn't quite make the grade either. Moffat, a mountain guide in her native Britain and a mystery writer, was mourning a friend's death, we're told, when her London publisher suggested she follow the pioneer trail to California--and her account of the journey is as casually personal and randomly suggestive as her opening. Intermittently, we hear about the several wagon trains on the trail in 1846, and periodically Moffat describes a difficult stretch they traversed, a spot where they made camp; now and again she actually sees wagon-tracks. But she is too independent a spirit, and too sophisticated a writer, to cast herself as a mere guide on a Heritage Tour. So we hear about her troubles with Old Crump, her four-wheel-drive wagon; and, very fleetingly, about people she meets, wildlife she sees, towns she passes through; the museum at Dinosaur National Monument, the geysers at Yellowstone; all manner of western lore; and myriad odds and ends--like a ranger's dead-serious advice ""to curl up in a foetal position, thus protecting the vital organs,"" should she meet a grizzly. Once, too, she takes a pack trip with a group--for a few focused pages. And it's when the Donner Pass looms at last (the third of the book's three maps shows just how the trails split, just which route each party took), that the book briefly coalesces around the pioneers' experience and Moffat's identification with them. But before and after, the randomness takes its toll. Moffat is consistently agreeable, simpatico company--picking up ""Haydn from Sacramento"" or preferring her sleeping bag to a proffered bed--and her ultimate tribute to the pioneers' ""fortitude"" gains from her own example and her British restraint. Thematically, however, that's an old story to most Americans, while the travel narrative is a very ragged thread (though it will undoubtedly be enhanced by her photographs).