Here is the moment in Daniel Mendelsohn’s book The Lost (HarperCollins) that stopped me cold, that touched a place so deep that I was trying to read on through a film of tears. Mendelsohn, a 40-something literary critic and classicist researching a family history, has traveled to Israel to interview yet another survivor of Bolechow, his grandfather’s shtetl.

Over tea and cake, he shows the woman, Anna Heller Stern, a photograph of one of her contemporaries, Frydka Jager, the daughter of Mendelsohn’s great-uncle Shmiel.

“Frydka var zeyer sheyn — zeyer sheyn!” says Anna. “Frydka was very pretty — very pretty!”

You should know that this is not a pivotal moment in a book packed with family history, personal confession, historical testimony, even Torah commentary. The Lost, in trying to flesh out the story of Shmiel and his doomed family — a wife and four daughters, or, as the subtitle has it, “six of six million” — is like one of those allegorical books in a short story by Jorge Luis Borges, vast and labyrinthine.

It is hardly the most dramatic or shocking moment in the book, adjectives that have to be reserved for the eyewitness testimonies of the survivors who watched their fellow Bolechowers marched to their deaths in mass graves or crematoria.

But in its sweet, mundane way, Anna’s comment illuminates Mendelsohn’s project, which is to shock the reader into recognizing himself or herself in the distant horror of the Holocaust. It recounts the almost 30-year search by the author, starting when he was a boy on Long Island, to construct a family history. While he manages to create a family tree that traces his ancestors back to the 18th century, it is the broken branch, represented by Shmiel, that seizes his imagination. It leads to a five-year quest to gather all that can be known about how his great-uncle lived and died.

Mendelsohn’s gift is the obsessiveness of his quest. Where most of us might have been satisfied to learn the birth dates and occupations of our distant relatives, perhaps gather a few scraps of memories about what kind of people they were, or read the letters they left behind, Mendelsohn wants more. Who were these people, really? Were they kind, selfish, flirtatious, cruel? In what did they delight, and what pissed them off?

His research takes him to Ukraine and to Australia and Israel. At each stop you expect him to say, “Enough. Got it. Shmiel was hard-working and slightly self-important, his wife Ester a fine balabusteh, his daughters lively and popular.”

But he presses on, and on, until his relatives emerge as something like real-life figures (Mendelsohn keeps reminding himself and the reader that most attempts to “know” people through the memories of their friends and acquaintances is ultimately futile). Secrets are revealed. It is a detective story, like J.B. Priestly’s An Inspector Calls, in which the detective builds a rock-solid case, only to see it crumble into disorder.

If, however, we can’t fully recreate the past, we can imagine. And by forcing us to imagine the everyday lives of the six, the book helps us comprehend the six million. “The dreadful irony of Auschwitz,” writes Mendelsohn, “is that the extent of what it shows you is so gigantic that the corporate and anonymous, the sheer scope of the crime, are constantly, paradoxically asserted at the expense of any sense of individual life.”

Perhaps that’s just a humanistic take on Stalin’s cruel maxim: “A single death is a tragedy; a million deaths is a statistic.” What the book feels like, actually, is a commentary on Mishna, the one that says, “When you save a life, you save the entire universe.” We can’t bring back Shmiel Jager or his daughters. Yet we can assert that each was an individual, with hopes like ours, and dreams like ours, and, before the horror began, the round of tedium and simple pleasures we call life.

“It had been to rescue my relatives from generalities, symbols, abbreviations, to restore to them their particularity and distinctiveness, that I had come on this strange and arduous trip,” Mendelsohn writes.

I was reading The Lost Saturday afternoon when, from the front stoop, I saw a family walking a pair of odd-looking dogs, great gray things that looked like trotting clouds. I thought my daughter and her friends would enjoy seeing them, and as I stepped inside, I called up to her bedroom, “Girls!”

At that moment something caught in my throat. I thought of all the life in that bedroom, of what it would mean to be separated from my children, of how blessed I was to be able to share something as ordinary as the sight of two extraordinary dogs. I heard the echo of an Australian Bolechower telling Mendelsohn how he used to call out to Shmiel’s youngest daughter. “She’d be playing in the backyard, I’d stay by the fence and say, ‘Hallo, Bronia.’ She was a sweet girl, still very childlike. You could see her mind was still on playing, on games.”

Recently I’ve been debating some Jewish activists, younger than Mendelsohn and I, who object to the emphasis on the Holocaust in Jewish education. They see the topic as too morbid, too despairing, as if the most important thing to teach about the Jews is how we die, not how we live.

Mendelsohn is not just interested in how “six of the six million” lived, but tells the story as if the very universe depends on it being told right and well. And in a sense, it does.