When Benjamin Franklin ran away from Boston, fed up with his older brother's dictatorial treatment of him as an apprentice at his newspaper, he left behind a large family that included Jane, his youngest sister. But no matter how involved he became in business, journalism, science and public affairs, Franklin never forgot to maintain his connection with Jane. Why he did so is the haunting story that Jill Lepore explores with pertinacity and patience.

This is a biography that almost did not get written because of the gaps in evidence about Jane's life and the biographer's doubts that a narrative could be fashioned out of the scant remains of a figure deemed unimportant by Franklin's early biographers — especially by Jared Sparks, who believed, in his 19th-century way, that only great men made history.

But Lepore took her inspiration from Virginia Woolf, who mused about what it would have been like to be Shakespeare's sister, a Judith who didn't have the opportunity to write as her brother did because women were not expected or, in most cases, were not allowed to do so. Instead, they were to husband their husbands, make a home and raise children — or, in Jane's case, many children.

Yet Franklin recognized in his sister not merely a family bond, but a woman like those in Samuel Richardson's novels, who had minds of their own. So he wrote to her and she wrote to him, often misspelling her meaning and apologizing for her meager education. He would not accept apologies, but saw in her words a wonderfully alert and independent sensibility. As she wrote after his death, "He while living was every enjoyment to me."

In Jill Lepore's wonderfully suggestive prose, Jane Franklin lives not merely as an individual rescued from obscurity, but as a character who brings to history and biography a new standard of measure, what the novelist Charles Brockden Brown called "a new kind of history," which would deal with those who "have no historian," but whose journals, letters and papers would show the "unjust prodigality of our sympathy to those few names, which eloquence has adorned with all the seduction of her graces."

Out of a little book Jane called her "Book of Ages," Lepore has re-created the life and the lineage of a personality worth knowing, one whose opinions her illustrious brother could not do without, given the pleasure they brought.

Carl Rollyson is the author of "Amy Lowell Anew: A Biography" and "American Isis: The Life and Art of Sylvia Plath."