No great American general has been so encrusted with pious cliché as Robert E. Lee. White-bearded, in impeccable gray cape and spit-polished boots, he stands in the national imagination in a posture of permanent nobility. In his portraits he seems forever posing for a monument. Whether astride Traveler, his equally noble horse, or standing in a doorway, hat in hand but with defiant gaze, in the great portrait by Mathew Brady taken a few days after Appomattox, Lee appears the embodiment of some ill-defined but powerful ideal. It isn’t merely “honor” or “manliness” or “duty and diligence” or even “dignity in defeat,” but some fuzzy amalgam of them all. The vagueness enhances the reverence his image inspires. At its most extreme, in a painting by the Southern artist Clyde Broadway, Lee appears as one member of a trinity composed of Jesus and Elvis, all three brightly haloed and surmounted by throbbing magnolia blossoms. The triptych, an icon of kitsch, is all the more ghastly for its poignancy.

The painting is reproduced in Elizabeth Brown Pryor’s brilliant “Reading the Man: A Portrait of Robert E. Lee Through his Private Letters” (Viking, 682 pages, $29.95). Ms. Pryor’s title is taken from Stephen Vincent Benét’s epic poem “John Brown’s Body,” in which he says of Lee, “And nothing helps us yet to read the man.” Ms. Pryor has taken this challenge literally. She reads Lee through his voluminous and little-known correspondence, the bulk of which remains — astonishingly enough — not in state or regional archives, but in the possession of Lee’s descendants. An acclaimed historian, whose earlier biography of Clara Barton remains the classic account, Ms. Pryor turns out to be a shrewd and astute exegete of this most enigmatic of men. Her reading strips away the distortions of adulation to lay bare a living man in his many terrible contradictions.

Ms. Pryor’s method is as original as her analysis. She begins each chapter with a letter or letters, from Lee as well as from his wife and family along with many others, and presents them without comment. The approach is disorienting at first but proves effective. Through it we become momentary eavesdroppers on long-vanished conversations. In her chapters, she then traces Lee and his family, from the American Revolution (in which his father, the rascally and heroic Lighthorse Harry Lee made his name) well into Reconstruction. The book becomes a kind of dialogue, not only between the correspondents who are quoted, but between the author and her subjects, between our century and theirs.

Ms. Pryor isn’t out to debunk Lee, but she doesn’t excuse him either. Of his fabled sense of honor, she notes that, “honor was bound up with family connections and local reputation and a desire to avoid public shame. For those who felt it keenly, the individual consideration of honor could take precedence over civic order.” Such a code led Lee to resign his commission in the military rather than find himself commanded to lead an invasion of Virginia at the outbreak of the Civil War. But Ms. Pryor notes too that “in military circles it was ‘dishonorable’ to resign because of unwelcome orders. Lee acted on this definition of honor at the very time he was ‘dishonoring’ vows of 30 years.” And, as she further points out, Lee was motivated as much by his resentment of the North and his hatred of Abolitionists as by lofty principles of personal honor.

The most disturbing chapters deal with Lee’s views on slavery. He thought slavery an evil system not because it stripped slaves of freedom and dignity but because it was such an awful burden on slave owners. For Lee, slavery formed part of some inscrutable providential design through which slaves might someday rise to a higher condition (though never to the level of whites). He was a brutal slave owner, destroying families to make a quick profit. “By 1860 he had broken up every family but one on the estate,” Ms. Pryor writes. Once he had a runaway slave given 50 lashes, urging his constable to “lay it on well,” and then had brine poured onto the victim’s flayed back — this was a slave who had been manumitted at Lee’s father-in-law’s death but whom Lee refused to free. Despite Ms. Pryor’s best efforts to put all this in context, Lee stands revealed as both cruel and hypocritical.

Lee had wit and grace in abundance, as his letters prove. And they display other unsuspected aspects of his personality. He was a lifelong flirt, indulging in startling sexual innuendo with female friends and relatives. He was a domestic tyrant who adored his children, lavishing them alternately with caresses and commands. But despite his considerable charm, something cold, some abstractly calculating tendency, characterized Lee. His troops regarded him as a father, but he let them be butchered by the thousands without so much as a backward glance; most conspicuously at Gettysburg, when even his own generals stood appalled. As Ms. Pryor shows throughout, Lee was simply unable to imagine the lives of others, whether slaves torn from their families or young soldiers squandered in suicidal charges. In crucial ways, Lee the man was more hollow — and more heartless — than the icon he became.