Only five letters survive from the correspondence between Martha and George Washington. The first first lady destroyed the rest – provoking this agonizing passage in Patricia Brady’s biography: “Of all the letters that Martha and George Washington wrote to each other over the years, the destruction of the correspondence of the spring of 1758 is most distressing. We have no idea of the tone, sentiments, or frequency of those courtship letters as the young people moved closer to a decision to marry.”

Martha Washington, that consummate conservator of her husband’s reputation, might well ask the biographer, “What business is it of yours?” Biography humanizes heroes, and what a new nation needed was a superman, not a flawed founding father.

How Martha would have hated the passages that follow, which discourse on George’s mooning over Sally Fairfax, his friend’s wife, “whose real purpose was to torment or reproach him about his coming marriage to the lovely widow Custis.” Martha had had a happy first marriage to an older, fabulously wealthy Virginia planter, who died leaving her the whole caboodle. In colonial Virginia both men and women expected to remarry, and the attractive Martha had suitors lining up.

This was no time for George, a planter of modest means, to dawdle. Yet Ms. Brady refers to him as “Poor George”:

His baffled, incoherent response the next day [to Sally Fairfax’s letter] shows just how young and emotionally vulnerable he was. He rushed to assure her “how joyfully I catch at the happy occasions of renewing Corrispondance which I feard was disrelished on your part. … In silence I now express my Joy. – Silence which in some cases – I wish the present – speaks more Intelligably than the sweetest Eloquence.

He wanted Sally, and she was quite willing to play with his emotions, but out of loyalty to her husband he withdrew his suit and turned resolutely to wooing Martha.

What goes on in a marriage is always something of a mystery to other parties. There are secrets that no one can ever quite fathom – even when that other George and Martha in “Who’s Afraid of Virginia Woolf” invite their guests to share them. (George and Martha’s disclosures are delivered with considerable hostility – “Get the guests!” George cries – for they share with their namesakes a sense that the rest of the world is full of interlopers.) The biographer is the interloper extraordinaire.

Here is another passage in Ms. Brady’s work that might have given even the stalwart Martha the vapors:

The New Kent County in which the Dandridges [Martha’s family] lived was pure country, fields bordered by forests, without a town worthy of the name. As a girl growing up on a small plantation, she had a matter-of-fact knowledge of sexuality, reproduction, and bodily functions. There was an earthiness to country life, with steaming manure heaps by the barn, chamber pots and privies, the fall slaughter of pigs and cows, the breeding of horses with bloodlines much discussed, the sounds of her parents’ lovemaking in the deep silence of the night, the birth of spring livestock – not to speak of the babies born on the place. Patsy [Martha’s nickname] never fell into the chilly, tight-lipped clutches of prudishness. Good-humored and laughing, she enjoyed all the pleasures life offered. Well, this is a lulling story, but what about the biographer’s confident use of words such as “never” and “all”? It reminds me of a contradictory phrase a student used to describe an early part of one of my biographies: “limited omniscience.” This is the biographer’s way of forcing the story. Was it that quiet at night in colonial Virginia? How close was Martha to her parents’ bedroom? I can understand a creaky bed and floorboards, but the other sounds? Just asking.

Even if you put aside the ethical question of prying into private life, what about the epistemological issue? What the biographer can know is, in part, a product of the language the biographer uses. In this case, where the biographer so often has to resort to “must have been,” “probably,” and “surely,” a better title for the book might have been “The World of Martha Washington,” a tipoff that Martha herself is rather an elusive character but that we can know a good bit about her by evoking her milieu.

Ms. Brady manages this feat quite splendidly. There are passages in this biography that just seem to exude the past and are a delight to linger over. Here is Martha on her way to join her newly elected husband, about to start his first term in the nation’s temporary capital, New York City:

Riding in a coach on unpaved roads was dirty work. Even with the curtains drawn, clouds of dust billowed in through the windows, settling on her traveling clothes until everything looked uniformly tan. Servants at Spurrier’s [Tavern] would have rushed to bring hot water for the ewer and basin in the room set aside for her. She needed to wash and change out of her grimy dress, its every pleat clogged with dirt. Oney or Molly [slaves] would have brushed the dust from her hair, combing it straight back from her forehead in her usual style, rebraiding the back, and tucking the tail in neatly. The maids would have opened a trunk, shaken out the wrinkles from a modish gown, and helped their mistress put it on. When Martha emerged looking considerably cleaner and more presentable, the coach and horses were ready and waiting at the door.

This kind of reclamation of the past has its own ethical imperative. As Montaigne emphasized, discovering what happened in the tent is a human event he prized more highly than understanding what happened on the battlefield.