In 1795 Thomas Jefferson was in the middle of a four-year vacation from public office. Having stepped down as secretary of state at the end of George Washington’s first term, and not yet elected as John Adams’s vice president, Jefferson was free to spend his time doing what he always claimed to love best: building and running his estate at Monticello. At the beginning of that year he wrote to his daughter Martha to discuss some business that her husband, Thomas Randolph, was attending to: the sale of two pieces of Jefferson’s property. “There remains on his hands,” Jefferson wrote, “Martin and the Chariot. If the latter cannot be disposed of without better wheels, I would be obliged to him to take the greater and larger diameters of the axle … that I may have a good set of wheels made here.”

Clearly, the chariot engrossed far more of Jefferson’s attention than the other item Randolph had to “dispose of”: Martin Hemings, the older half-brother of Sally Hemings, Jefferson’s lifelong concubine and the mother of seven of his children. This offhand reference, writes Annette Gordon-Reed in her new book, “The Hemingses of Monticello” (W.W. Norton, 800 pages, $35) “says all that needs to be said about what slavery was and meant to the people who lived under it.” In any society that we can call sane or decent, Martin Hemings would have been Thomas Jefferson’s brother-in-law; in 18th-century Virginia, he was his property. When the relationship between the men went bad and erupted in a quarrel, as seems to have happened sometime in 1792, Jefferson did not have to make up with his relative or pretend to like him or even figure out a way to avoid him. Instead, he sold him. As Jefferson explained, “Martin and myself disagreed when I was last in Virginia insomuch that he desired me to sell him, and I determined to do it.”

Yet as these very lines make clear, Jefferson’s relationship with Martin Hemings was not the ordinary one between master and slave in the antebellum South. As Ms. Gordon-Reed points out, “Jefferson could have forced Hemings to continue at Monticello and, if required, had him beaten into submission.” Instead, as so often in the story Ms. Gordon-Reed has to tell, Jefferson tried to come up with some alternative to simply wielding his legal power. After all, the request to be sold came from Martin, not Jefferson, and the master even allowed his slave to pick his new owner: “I will subscribe to any … master whom he shall chuse.” We can almost imagine that Martin Hemings was a dissatisfied employee putting in for a transfer.

The central argument of Ms. Gordon-Reed’s book, however, is that nothing in the peculiar, complex relationship between Thomas Jefferson and the Hemings family can palliate the evil at its root. The reason Martin and his relatives were treated so leniently by Jefferson had nothing to do with justice. It was, rather, the love that Jefferson bore for Sally Hemings that won her mother, siblings, and children the special status they enjoyed at Monticello. And as Ms. Gordon-Reed shows in this morally painstaking study, that love — which is the very reason we know the name of Sally Hemings, while the names of so many African-American slaves have been forgotten — is itself poisoned by the inequities of slavery. Is it even possible to apply the word love to a relationship between a woman and the man who owns her? Can we ever know what was in Jefferson’s and Hemings’s hearts, given the cloak of secrecy that they and their society conspired to draw around their relationship?

It is the very unanswerability of such questions that makes “The Hemingses of Monticello” such a long and, despite its challenging subject, undramatic book. Ms. Gordon-Reed, a professor of law at New York Law School and of history at Rutgers University, is not simply telling the story of “an American family,” as her subtitle calls it. If she were, her book would not be one-tenth as long: There is simply not enough known about the Hemingses to fill out their story much beyond names and dates. Like almost everyone, black and white, who lived in 18th-century America, the Hemingses left no letters or diaries that would allow us to understand their lives from within.

What we do know, however, is substantial enough. The family traced its origins to the union of an English seaman, Captain Hemings, and a “full-blooded African” woman, possibly named Parthenia, who was owned by Francis Eppes IV of Virginia. Their daughter, Elizabeth Hemings, was born around 1735 and would live an unusually long time, dying at Monticello in 1807. Elizabeth, the matriarch of the Hemings clan and the real heroine of Ms. Gordon-Reed’s book, was given to Martha Eppes, Francis’s daughter, upon her marriage in 1746. Her husband was named John Wayles, and their daughter, Martha Wayles, would become Thomas Jefferson’s legal wife.

At the same time, John Wayles fathered six children with Elizabeth Hemings (she also bore eight other children by other men). One of those six was Sarah Hemings, nicknamed Sally, who would accompany Martha Wayles upon her marriage to Jefferson. Sally Hemings, then, was her mistress’s half-sister. Jefferson would have long relationships with both sisters and father children with both of them; but only Martha, who was white, could be his legal wife. Sally, who had three white grandparents, was legally black, and a slave. As a result, Ms. Gordon-Reed points out, she could never even be Jefferson’s common-law wife, although the two of them spent 38 years living together at Monticello — far more than the 10 years Jefferson spent married to Martha before she died.

Even worse, while Jefferson was clearly concerned with the future of his children by Sally Hemings — naming them after his friends, teaching them trades, and finally granting them freedom — he was, legally, not their father but their owner. His children by Martha inherited his name and property; his children by Sally were slaves, whose very existence had to be hidden from the prying eyes of Jefferson’s political enemies. Ms. Gordon-Reed invites us to consider the feelings of young Madison Hemings, Jefferson’s son, who was named for his great friend James Madison: “Imagine a six-year-old with a father who is always kind to him, but who seems to ration his affectionate gestures, viewing that same father freely expressing his affection for his [legal, white] grandchildren, who were around the six-year-old’s age.”

Of course, Ms. Gordon-Reed finds it nearly impossible to imagine — just as it is impossible for us to know how Sally Hemings felt about Jefferson, and vice versa. What really accounts for the size of this book is that it is built around such deadlocks: failures of communication and understanding across the generations. Ms. Gordon-Reed’s greatest strength is her sense of justice, unimpaired by piety toward a founding father. Reading “The Hemingses of Monticello,” it is impossible to ignore the vanity, complacency, and racism that allowed the author of the Declaration of Independence to treat the Hemingses as he did. But her sense of historical irony, of the infinitely contradictory and perverse ways human beings can live together and treat one another, is less vivid. “The Hemingses of Monticello” is better at judging the past than at entering into it, which is why it is an easier book to admire than to enjoy.

akirsch@nysun.com