Marks never quite settles on a comfortable tone sincerity or kitsch? for her ''biography'' of the famed fictional spokeswoman for Gold Medal Flour Finding Betty Crocker. (Twenty-four years after her ''birth'' in 1921, Betty Crocker was named the second-most-popular woman in America after Eleanor Roosevelt for her contributions to domestic tranquillity and efficient home-making.) And sometimes Marks' transitional commentary sits like a bread loaf missing yeast. (''Some listeners clustered around the proverbial back fence to exchange gossip. . . .'') But the heartfelt letters Marks has collected, written with good manners by real people asking for real help and receiving it from the General Mills staff representing Ms. Crocker, are extraordinary and moving a testament to trust well placed.