UNTIL well into middle age, Austin Dickinson's life as a lawyer and pillar of Amherst society resembled that of his bardic sister, Emily, "too simple and stern to embarrass any." Then on the night of September 10, 1882, he invited the young, multi-talented Mabel Loomis Todd to sing at the Dickinson Homestead. Only he and his younger sister Lavinia actually saw the performance, for his wife Susan was estranged from the family, his mother was bedridden, and the reclusive Emily elected to listen from a dark hallway. As the music died away, the poet sent the visitor a glass of sherry and two prophetic verses:

Elysium is as far as to

The very nearest Room

If in that Room a Friend await

Felicity or Doom What fortitude the Soul contains,

That it can so endure

The accent of a coming Foot--

the opening of a Door--

The following night Austin escorted Mrs. Todd to a whist party at Evergreens, his own house next door. Deep in conversation, the couple stopped, gazed into each other's eyes, and confessed their love. Austin was 54, Mabel 26. They had been acquainted less than a year.

For Austin Dickinson, an aristocratic figure with an improbable shock of coppery hair, and for Mabel Todd, with her bright eyes, provocative mouth, and slim-fitting, hand- painted dresses, the door had indeed opened onto a new threshold of romantic possibilities. Austin noted the event in his diary with one word--"Rubicon."

Three months later, while David Todd, professor of astronomy at Amherst College, was in California (appropriately recording the Transit of Venus) his wife consummated her love for Dickinson in the dining room of the Homestead. As the liaison flourished, Mabel continued to love and make love to both husband and lover, recording each occasion in her diary. In 1884, for instance, she made love with David an average of eight times a month, and with Austin, 12. Approximately half of these couplings consisted of full intercourse crowded into the last 10 "safe" days of Mabel's menstrual cycle.

The affair became common knowledge among the townsfolk, including Mabel's adolescent daughter Millicent. She recorded in fragments of autobiography (deposited at Yale, but not cited here) feeling "degraded" and "disinherited" when her mother began wearing Austin's rings on her left hand and David Todd's on her right. Millicent was also aware of her father's remarkable collusion, observing that as he returned late from the laboratory, he would whistle a tune from Martha ("M'appari"?) to warn the lovers of his approach. His indulgence--even to the point of watching Mabel and Austin in action--is more understandable when we learn that he seduced most of the female guests who came to his house, including Millicent's friends.

At the time the illicit relationship began, the Dickinson parents were dead and the Loomises living in Washington, D.C. Austin had been married to Susan Gilbert, an intellectually brilliant but sexually frigid woman with a morbid fear of childbirth, for 26 years. In spite of several abortions, she had produced three childen, one an epileptic.

Emily, who always scoffed at "plated wares," smiled on the new lovers' genuine emotion. The peppery Lavinia, a frequent victim of her sister-in-law's high-handedness, became a willing accomplice, allowing Austin to use her mail box for Mabel's correspondence.

During the 13-year affair, the couple exchanged over 1,000 letters, some 250 of which are reproduced here. Repeatedly the mystical nature of the relationship is sounded. "I have found in you what a woman may be to a man, hope, courage, joy, inspiration, rest, peace, religion. . . . You are my Christ. God reveals himself to me through you, and in you." Mabel reciprocated. "You are my gate to God--the high priest of my best self."

For some tastes, there may be an excess of rapturous letters in Austin and Mabel. They take up over half the book, which begins with two chapters on Todds and Dickinsons and ends with an acrimonious lawsuit.

Polly Longsworth is a very professional writer and editor, but a straightforward biography of Mabel, who was by far the most interesting of her four chief characters, would have given the story a more cohesive structure and simpler narrative flow. Too many lengthy footnotes clutter the pages. As Henry Adams pointed out, what is not worthy of inclusion in the text is probably best omitted. But this is a poignant enough saga to satisfy most readers as it moves toward its tragic d,enouement.

While Mabel's dream of becoming Austin's wife and bearing his child faded (she abandoned the use of "safe" periods with him for all of 1888), she added to her accomplishments as musician, painter and writer that of editor, --assembling and publishing several volumes of Emily Dickinson's poems and letters. She also embarked on a brilliant career as a lecturer, giving as many as 30 talks in the 1895 season alone.

But the year that was so satisfying professionally brought her bitterest personal blow. On August 16 Austin Dickinson, worn out by overwork and domestic turmoil, died. Mabel could not be consoled. "The whole town weeps for him," she wrote. "Yet I am the only mourner. . . There never was such love as his for me and mine for him."

Throughout her remaining 37 years (David was institutionalized in 1922), the pain of Austin's death never ceased. "Nothing on earth has power to move me any more. One old brown coat and big hat used to be enough to set every drop of blood in my body tingling and racing through the veins in tumultuous rush." As Emily once put it: "The Heart wants what it wants--or else it does not care."