Laura Pomfret, 43 and L.A.-suburban and mildly discontented, goes, all aflutter, to her 25th highschool reunion--in a chatty memoir that swings likably, but uneasily, from Bombeck-ian satire to dippy farce to limp attempts at something a bit more serious. ""Would Archduke Ferdinand make a sentimental journey to Sarajevo?"" That's Laura's first reaction to the reunion invitation. After all, she became semi-notorious back home in Minnesota when--jilted by her gorgeous deflowerer, Lanny Merkel--she painted his name on the town's dozens of religious ""REPENT"" signs. Moreover, despite nice husband David and three precociously cute (too cute) kids, Laura is dissatisfied with her life, her lack of career and fame. Still: ""The thing to do, of course, was go back and face the old bete noire, if I got the beast to heel. . . I could get on with the mid-life crisis and search for my shy talent."" So Laura diets off ten pounds, searches for ""The Outfit"" in cartoony L.A. boutiques (she'll wind up wearing a beige satin slip-dress bought for $1 at a garage sale), ignores the doomsday warnings of her mother and jargon-spouting pal Mary Jean. . . and takes off alone for Minnesota (professor David has a seminar in Spokane). But will Lanny be at the reunion, gorgeous as ever? Yes indeed. And though accompanied by an insufferably perfect wife (""Olga was so evolved I suspected we were not actuary of the same species""), Lanny immediately begins seducing Laura. . . who had half-intended to seduce him as part of some oblique revenge scheme. So, after pallid reunion festivities and some bedroom-farce turns, a night of illicit passion does ensue--but the switch to seriousness here (""The argot of lovemaking became a soft lamentation"") is unconvincing, as is the casual implication that this little spot of adultery will help still-happily-married Laura in her search for self-expression. When presuming to deal with a real mid-life crisis, then, first-novelist Sullivan falls flat. When playing strictly for laughs--with more than a few bitchily on-target recognitions--she's fairly pleasant, if highly unoriginal, company.