Readers glancing at that title may hastily conclude that no couple named Austin and Mabel could ever rank among history's great lovers--and, alas, they don't. Austin Dickinson (1830-1895), a lawyer and leading citizen of Amherst, was comfortably installed in oblivion, and Mabel Loomis Todd (1856-1932) was remembered only for her pioneering but bad edition of the work of Austin's sister Emily, when their love letters arrived at Yale in 1968. These provided some grist for the mills of Professor Richard Sewall (in his two-volume life of Emily Dickinson) and Peter Gay (in his forthcoming The Bourgeois Experience, p. 1158); but Longsworth (one Sewall's assistant) is the first one to attempt a full-dress presentation of the affair. Mercifully, she has spared us the bulk of their effusions, printing only a quarter of their more than 1,000 surviving letters. But Austin and Mabel were simply not very stimulating writers. (""Oh I love you thrillingly,"" she cries, ""Do you know it? Don 't you know it!"" ""I love you,"" he replies, ""I admire you, I idolize you. I am exalted by your love for me."") Their passion was too monotonously happy (until it ended, after twelve years, with Austin's death), and their whole world too narrow and complacent. Still, Longsworth's exhaustively researched story has its intriguing features. Mabel's handsome young husband David, an astronomer at Amherst College, actively supported her adultery, while Austin's wife Sue bitterly opposed it. Mabel seems to have been looking for her father in the late-middle-aged and not especially prepossessing Austin, who welcomed her love with understandable gratitude. Mabel could be silly and vain, yet she jolted staid little Amherst by her fierce public mourning for her lover. The Dickinson-Todd correspondence may be a marvelous document for the social historian, but for the general reader it's a minor curiosity.