His concubine was also his wife's half-sister.

This is not a banal stab at unmasking the biases of American history, however. Of course the Hemingses lived lives constrained by the social categories imposed on them: They remained slaves in a slave society, and black enough in a nation committed to white supremacy. But the book's subject is not the categories themselves; Gordon-Reed doesn't use the Hemings family as a metaphor for the "black experience," or Sally Hemings to humanize "slave women." She focuses instead on the individuals who struggled messily to survive despite these categories and, every once in a while, broke through them.

The result offers unparalleled insight into an 18^th-century Virginian world in which rigid racial boundaries were impossible to police. Interracial relationships raised few eyebrows, and John Wayles' political career never suffered from his concubinage with Elizabeth Hemings. One of the Hemings sisters, Mary, was leased to a local white merchant, Thomas Bell, and the two became lovers. At her request, Jefferson sold Mary to Bell, and the couple lived openly in what was, in effect, a common-law marriage; their children quietly became free by sanction of the community, if not the law. In 1802, when newspaperman James Callendar first reported Jefferson's relationship with Hemings, his account suggested how widely known their relationship was in local circles, where Hemings appeared like "something like a wife to Jefferson."

The Hemings story also casts new light on Jefferson, portrayed as an agonized hypocrite in much recent scholarship. He emerges instead as a man whose life can be fully understood only through its relationship with slavery. The private Jefferson, in Gordon-Reed's reading, did not simply express his fascination with human cultivation and control through the architecture of Monticello, but even more through his relations with the Hemingses and his other slaves. Though he could never see them as "separate from his own needs, desires, and fears," they nevertheless recast his life: The Hemingses gave him "a beautiful younger mistress and children, who could be shaped into some version of his private self—woodworker, musician, and sometimes gardener."

Jefferson's mixed-race children remained in the United States, fusing into the white community seamlessly: an eloquent rebuttal to Jefferson's public denunciation of racial mixture and his endorsement of blacks' expulsion from America. Alas, his words carried more weight than his actions in a nation increasingly committed to slavery and racism, and Jefferson's descendants and biographers sought to redraw the categories his life had blurred, writing the Hemingses out of history in a quest, as Gordon-Reed puts it, "to maintain the ownership over black people's identities in perpetuity."

There, perhaps, lies the fullest significance of this remarkable family's story: It makes that perpetual ownership impossible.

Correction, Sept. 24, 2008: François Furstenberg originally referred to Annette Gordon-Reed as a professor at New York University. She is a professor at New York Law School and a member of the history faculty at Rutgers. (Return  to the corrected sentence.)