Little seems to have been held back. The still smoking World Trade Center ruins have been juxtaposed with a shot, by Ms. Sontag, of Ms. Leibovitz, naked three weeks later, on the day before she goes in for a Caesarean section. Her mother, sister and niece lie, intertwined in grief, on a bed where her father has died hours before. Ms. Sontag’s body rests on a table in a funeral home, decked out valiantly in a pleated dress from Milan.

But it is the photographs of Ms. Sontag, taken in a hospital room at the Fred Hutchinson Cancer Research Center in Seattle a month earlier, barely recognizable but unmistakably dying, that Ms. Leibowitz says proved the most contentious in conversations with friends and family about making the pictures public.

“Let me be very, very clear about this,” she said in a long conversation in her studio in Greenwich Village, during which she alternated between speaking openly about intimate corners in her life that the photographs inevitably expose, and seeming to regret having said anything at all. “Every single image that one would have a possible problem with or have concerns about, I had them too. This wasn’t like a flippant thing. I had the very same problems, and I needed to go through it. And I made the decision in the long run that the strength of the book needed those pictures, and that the fact that it came out of a moment of grief gave the work dignity.”

It’s a complicated question.

“You don’t get the opportunity to do this kind of intimate work except with the people you love, the people who will put up with you,” Ms. Leibovitz said, speaking not just of Ms. Sontag but of her parents, her children, her five brothers and sisters, who she says became one another’s best friends, growing up in a military family perpetually on the move. “They’re the people who open their hearts and souls and lives to you. You must take care of them.” But when she began sorting through 15 years of magazine assignment photos and personal photography in August 2005 for a book she had long ago promised her publisher, the personal pictures were the ones that captivated her. She would weep for 10 minutes, then return to the photographs. “I found myself totally taken over by the personal work,” she said. “I thought it was so strong and so moving.”

She spent five days that month in the complex of stone barns she owns on 200 acres in Rhinebeck, N.Y., working on the book with Mark Holborn, an editor and publisher who has collaborated with other photographers on their books. They tore down the divide between Ms. Leibovitz’s photographs that had been taken on assignment and her personal images, interweaving them in one narrative spanning 15 years in the world and her life. Her landscape photographs became the punctuation, “pauses and commas in the storytelling.” At the end of five days, she said, “it was the first time in my life you know you have something that is good or important or that matters.”

Yet Ms. Sontag was a private person, Ms. Leibovitz said: “If she was alive, of course this work wouldn’t be published. It’s such a totally different story that she is dead. I mean, she would champion this work.”

Ms. Leibovitz herself seems ambivalent about how much to surrender. She resisted including an introduction; the photographs were “a small movie,” she said; they should speak for themselves. An editor, Sharon Delano, convinced her that “it was important to explain myself, and explain myself once,” Ms. Leibovitz said. An 11-page introduction grew out of several months of conversations with Ms. Delano. “She was absolutely right,” Ms. Leibovitz said. “I love the introduction. It’s just like a clearinghouse for myself.”

But who gets to explain themselves just once these days? The introduction is a gem of lucidity and understatement. (Ms. Leibovitz introduces the fact of her relationship with Ms. Sontag as an aside, in a dependent clause: “who was with me during the years the book encompasses.”) Now the book is coming out, and she is called upon to talk. “This gets personal,” she said, stopping herself one of several times during the interview. “I have to save some of it for myself. I’m trying to figure this out.” You have to become an actor, she complained; but you don’t want to stop feeling. “In the long run I don’t think this book is helped by talking about it,” she said. “I worry about talking about it. Here I am talking about it.”

Ms. Leibovitz, who is 57, made her name in the 1970’s and early 1980’s as chief photographer for Rolling Stone, shooting musicians and others in provocative poses, like John Lennon, naked and pink, curled around Yoko Ono, fully clothed in black, just hours before he was killed.

In 1983 she became the first contributing photographer for Vanity Fair. She shot the famous 1991 cover photograph of Demi Moore, naked and pregnant. She was the official photographer for the 1996 summer Olympics in Atlanta. Other work has included photographs for advertising campaigns for American Express, the Milk Board, “The Sopranos.” Those photographs tend to be meticulously composed, humorous and in strong, saturated color.

She shot her personal photographs with a 35-millimeter camera, sometimes a Leica, in black and white, with Tri-X film, the way she started at the San Francisco Art Institute in the late 1960’s. There are photographs of her parents, her siblings, a flock of nieces and nephews on the beach; of room-service breakfast with Ms. Sontag at the Gritti Palace in Venice; of her parents, asleep in bed, elbows akimbo and pillows askew, a small grandson sandwiched in between. She never took a lot of personal photos; she would throw a few rolls in a box, let them go undeveloped for months. Ms. Sontag complained she did not take enough.

They met in the late 1980’s when Ms. Leibovitz was asked to make some publicity photos in connection with the release of Ms. Sontag’s book, “AIDS and Its Metaphors.” As a student she had read “On Photography,” in which Ms. Sontag wrote, among many other things, “To photograph is to appropriate the thing photographed.” But she talked to Ms. Sontag about “The Benefactor,” her first novel, which Ms. Leibovitz loved. Then she went out and bought all of Ms. Sontag’s books. “I remember going out to dinner with her and just sweating through my clothes because I thought I couldn’t talk to her,” she said. “Some of it must have been I was just so flattered she was even interested in me at all.

“She was actually a very warm, outgoing person, the opposite of what you sort of expected — just so charming, even childlike in some ways,” Ms. Leibovitz said. “The early pictures of her riding the bike, when she had her bike. I helped her get her driver’s license. I said, ‘Oh my God, what did I do?’ Because I realized she couldn’t really drive. She was just this charming, beautiful child inside. She had such delight with life and everything.” Ms. Sontag told her, she said, that as a photographer, “you’re good but you could be better.” Ms. Leibovitz wanted to be better. “I think she came into my life at the right time,” she said. “I wanted to do better things, take photographs that matter.”

They traveled together in Jordan, Egypt, Italy, Paris. Ms. Leibovitz went to Sarajevo, which she says she would not have done had she not known Ms. Sontag. They talked about Ms. Leibovitz doing a book Ms. Sontag called the beauty book: photographs she would take when she felt moved to take them, not the kind taken for assignments. Ms. Leibovitz bought an apartment in Paris; she wanted Ms. Sontag to have something she had always wanted. She helped her financially, she says, making it possible for Ms. Sontag to stop doing lectures and concentrate on writing fiction. “We took care of each other,” she said. “I had great respect and admiration for her, and I wanted to make everything possible for her, whatever she needed. I felt like a person who is taking care of a great monument.”

Another person who emerges vividly from the photographs is Ms. Leibovitz’s mother, Marilyn, whom she describes as exuberant and creative, the kind of mother your friends like but you find occasionally embarrassing as a child. She appears often in a bathing suit and in the presence of water: a stocky figure wading into the surf, grandchild in tow. On her 75th birthday she is barefoot at the sink in a small kitchen in the Florida Keys. Her broad back is turned. Her husband, Samuel, sits, at a tiny table, his back turned too. “That’s what it was really like growing up,” Ms. Leibovitz said, laughing. “You saw their backs. That’s such a moment in life. It makes me a child again.”

In 1998 Ms. Sontag received a diagnosis of cancer, from which she recovered. Ms. Leibovitz took several months off to be with her. There are photographs of that period too, of Ms. Sontag receiving chemotherapy, having her hair cut. “You know, one doesn’t stop seeing,” Ms. Leibovitz said, when asked about her impulse to photograph illness. “One doesn’t stop framing. It doesn’t turn off and turn on. It’s on all the time.” In the middle of her Caesarean in 2001 she reached up with a camera to try to shoot the birth of her daughter, Sarah, over the curtain suspended across her midriff. “They’re all totally out of focus and terrible,” she laughed.

She photographed her father after his death in 2005. He was 91, had lung cancer and had driven a car until a week before. He died at home in bed, with hospice care, in his wife’s arms. The family kept his body in the bedroom all day, as children and, later, a rabbi arrived. Ms. Leibovitz photographed him there, his head on a flowered pillowcase, in pajamas with dark piping. “You find yourself reverting to what you know,” she said. “It’s almost like a protection of some kind. You go back into yourself. You don’t really know quite what you’re doing. I didn’t really analyze it. I felt driven to do it.”

She said, “My father was so beautiful lying there.”

A year earlier, when Ms. Sontag became ill for the final time, Ms. Leibovitz stopped shooting. “I didn’t want to be there as a photographer,” she said. “I just wanted to be there. Then, at the very end, I forced myself to take those few pictures. I knew she was probably dying.” Ms. Leibovitz had Ms. Sontag flown by air ambulance from Seattle back to New York.

“Susan really fought for her life,” Ms. Leibovitz said. “I don’t think anyone takes in what that means. She had a very slim chance of the bone-marrow transplant working. She wanted to live. She had more books she wanted to write. She wanted to do more. She did not want to die. I think it was a very brave and courageous year of her life.”

Near the end of the book there are photographs of grandchildren shoveling dirt onto Mr. Leibovitz’s’s grave and of the birth of Ms. Leibovitz’s twins, Susan and Samuelle, by a surrogate mother. There are photographs of Mrs. Leibovitz, in July 2005, with her two sisters, eating outdoors in Rhinebeck, a cane across her lap, a small camera in hand.

“I love that picture,” Ms. Leibovitz said. “It’s just, life goes on. She’s gone back to the support of her sisters.”

The book and the exhibition end with landscapes, images of Monument Valley, the Hudson, the view from Ms. Leibovitz’s apartment in Manhattan toward Ms. Sontag’s after snow. In one, on Mount Vesuvius, Ms. Sontag moves away from the camera, ascending toward a ridge. Asked about the placement of the final images, Ms. Leibovitz said, “It’s a way of moving out of the story, it’s a way of going back into the earth.”

In the end how much more needs to be said, really?

“With Susan it was a love story,” Ms. Leibovitz said. “With my parents it was the relationship of a lifetime. And with my children it’s the future. I just tried to create an honest work that had all those things in it.”