As a boy in Old Bethpage, Long Island, in the 1960s, classicist and literary critic Daniel Mendelsohn was entranced by his grandfather Aby Jäger's stories about the Old Country, long, digressive tales told with humor and dramatic flair, whether about Aby's own torturous journey to America from Bolechow (a shtetl at the foot of the Carpathian Mountains in what is now Ukraine) or the sister who was forcibly betrothed to an ugly cousin, but "died a week before her wedding."

Decades later, sitting in a plush chair in his small apartment on Manhattan's Upper West Side, Mendelsohn says, "I remember when I was 12 years old typing out my grandfather's stories on an old Underwood typewriter that weighed 600 pounds." Those stories were his inspiration for becoming a writer and for his forthcoming book, The Lost: A Search forSix of Six Million (HarperCollins, Oct.).

The book is Mendelsohn's exploration of the family story that remained veiled in mystery: that of Aby's brother Shmiel, who had stayed in Bolechow and, with his wife and four daughters, was "killed by the Nazis." Mendelsohn overheard in whispered snippets that the family had been betrayed after hiding in what he believed to be a castle. Only later did Mendelsohn learn that it was a Yiddish word he had heard for where they had been hidden.

After Aby Jäger's death in 1980, Mendelsohn discovered some letters, dating from the late 1930s, in which Shmiel pleads with Aby for help in getting out of Europe. "Whenever you read a letter, you feel implicated," Mendelsohn says. "It's so immediate. It's you and the voice of this person saying, 'Help me, get me out.' To be confronted with these documents was like being yanked by the neck." The letters launched Mendelsohn on the remarkable journey of discovery, or more accurately, of recovery, that he describes in The Lost. He traveled to Bolechow, Israel, Scandinavia and Australia to reconstruct the life, and death, of Shmiel and his family.

Stories have remained central to Mendelsohn, both as a scholar and as a literary critic who has written for the New York Times Book Review and the New York Review of Books, among other publications. But for him, The Lost is more than just a story: he speaks of it as a moral responsibility, emphasizing passionately the need to preserve a "fragile" past. "I went to family seders and bar mitzvahs, and there were people with [concentration camp] tattoos on their arms. But my kids will never know a person like that. Never. So how do I give this to subsequent generations? How do I wrap this up in a package that makes them sense that?"

Mendelsohn thinks of The Lostas part of a trilogy, each book expressing one aspect of his identity and life. His first book, The Elusive Embrace (Knopf, 1999), looked at his experiences as a gay man, at issues of "identity and pleasure and responsibility and children." (Mendelsohn serves as a father figure to the two sons of a friend, spending half of every week with them.) And he is thinking of a third book that will draw on his classical studies to examine contemporary culture.

Mendelsohn's own view of the world has been shaken up by his travels. "I think of eastern Europe as a giant cemetery now, which isn't how I used to think of it. Every town you go to, someone will say, 'Oh, do you want to see the mass grave?' You realize how easily civilized values and behavior fall apart under stress."

Mendelsohn says he was a "nerdy, scholarly" kid, obsessed with the past, both familial and historical. His brothers and sister "used to make fun of me for spending my summers in cemeteries," he says. A "manic reader," he wasn't interested in the biblical stories he learned in Hebrew school. "When you are... a certain kind of child—odd, perhaps; perhaps the kind of child whom other, bigger kids pick on—you do not want to spend your time reading about victims....," he writes in The Lost. Instead, Mendelsohn read about the ancient Egyptians and Greeks; he dressed up in pharaonic regalia and kept his diary in hieroglyphics.

In love with the ancients, Mendelsohn obtained his Ph.D. in classics from Princeton in 1994. But he had always wanted to be a critic. "That sounds so funny as a thing to aspire to. When I was in high school, in a great era of the NewYorker, I read Andrew Porter on music, I read Pauline Kael on film, and I just loved criticism. And when I was in college I would write these faux NewYorker reviews." In graduate school he began writing for a weekly campus paper, and one of the editors, Ariel Kaminer, said, "You know, people will pay you do to this." And Mendelsohn said, "Really?"

Mendelsohn doesn't see himself as a writer first. "I never think of criticism as an ancillary or secondary activity. I think it's a very important thing. Particularly now in this Internet culture, where anybody can essentially publish any opinion about anything, all the more reason to have people who know what they're doing do it."

And in The Lost, he returns to the biblical texts he scorned as a child, the early chapters of Genesis that tell of the creation, Cain and Abel, the flood and the story of Abraham. He weaves them into his own narrative, while also drawing on two commentators, the medieval scholar Rashi and the contemporary Rabbi Richard Elliot Friedman, bringing out thematic parallels. It wasn't part of his original plan for the book, but, he explains, "I was writing about this very difficult emotional material about brothers and siblings," as he wondered what Aby's response to Shmiel's pleading letters had been, why he wasn't able to help him get out of Europe. "I just thought, I'll look at Cain and Abel, because that's the ur-text of brothers not getting along." From there, he began to see relevant themes emerge in other Genesis stories.

This biblical study, and the research for The Lost, have given Mendelsohn a new appreciation for his heritage. "All these people were wiped off the face of the earth because they were Jewish. And as a secular, suburban High Holy Days Jew. I was always interested in the idea of Jewishness, because I was interested in family history. But the actual substance of this religion, these texts, was the reason these people were killed, and I thought I had to deal with it. I felt I owed it to them, although they weren't religious, that I had to say I paid attention to this for at least a little while."

Just as The Lost took root in a story that wasn't told, it refers to other tales that Mendelsohn can't tell, like the wartime experiences of Meg Grossbard, a Bolechower who survived the Holocaust and whom Mendelsohn located in Australia. "No one knows how Meg got through the war. She has never told a living soul, as far as I know. I get goose bumps telling you this because it's very strong, and it's poignant, especially in this day and age, where everybody is telling their story, every schlub who ever brushed his teeth in the morning can be on TV. We feel 'entitled' [to know their stories]." But, Mendelsohn emphasizes, "It was their life and it doesn't necessarily belong to us." And, he quotes Meg Grossbard, "Some things can't be told."

And, as he notes repeatedly in The Lost, many things remain unknowable, such as Aby's response to Shmiel's letters. Mendelsohn is haunted by how fleeting such information is. "Half of the 12 Bolechow survivors I interviewed died between when I started looking for them and when I finished the book," he says. The Lost is his powerful memorial to them and to their vanished world.