The genial but pallid debut of a new sleuth: widow Mrs. Eugenia Potter, a pale, saccharine Miss Marple imitation who's sleuthing here in her Iowa hometown, where (home for her annual visit) she has organized an adult-ed cooking-class with noted gourmet James Redmond. Within hours after the first class, however, beautiful young Jackie Morsback is found cut-throat-ed (with the knife from the cooking class) and Mrs. P.'s old beau Mackay Moore is found dead too--an apparent suicide. Plus: one more body will also surface (yet another cooking-class student)--as Mrs. P. goes through her long list of suspects over and over again, writing out hypothetical motives and murder scenarios. Unfortunately, however, the actual solution is no more plausible or satisfying than the dozen alternative theories. And Rich lays on the folksiness relentlessly: cute family nicknames, old high-school memories, twin policemen, stilted dialogue, platitudes, and sentimentality galore. Weak mystery, then, with trimmings that will please only the most sedate, old-fashioned readers; but the cooking details, from recipes to knife care, may engage some of the overlapping gourmet/mystery audience.