Diane Keaton’s book about her life is not a straight-up, chronological memoir. It’s a collage that mixes Ms. Keaton’s words with those of her mother, Dorothy Deanne Keaton Hall, who died in 2008. Since Ms. Hall left behind 85 scrapbooklike journals, a huge and chaotic legacy, there is every reason to expect that Ms. Keaton’s braiding of her own story with her mother’s in “Then Again” will be a rambling effort at best.Instead it is a far-reaching, heartbreaking, absolutely lucid book about mothers, daughters, childhood, aging, mortality, joyfulness, love, work and the search for self-knowledge. Show business too. The collage format works so well for Ms. Keaton that she can easily weave her love affairs with three very famous film luminaries into the larger tapestry of her life with family and friends.Not many lives would lend themselves to this kind of autobiographical treatment. But Ms. Keaton’s timing is so different from her mother’s that the contrasts between their lives are full of drama. “At 63, I’m doing what Dorothy did when she was 24,” writes Ms. Keaton, who adopted the first of her two children when she was 50. Dorothy was a young housewife when she raised Diane and her three younger siblings (two sisters and a brother). And when they left home, they left her at a loss, and the journal writing began.“At 54 Dorothy was put out to pasture with 32 more years of living staring her in the face,” Ms. Keaton writes. “At 65 there is no pasture, and I’m not lonely.” Ms. Keaton, the la-de-da loner for so much of her life, now finds herself thrust “out of a life of isolation into a kind of family-of-man scenario, complete with an extended family, new friends and much needed ordinary activities.” Her mother never had the option of a third-act rescue.It is terrifically poignant to learn that Ms. Keaton, whose public personality was that of someone so charmingly addled, had to deal so seriously with her mother’s mounting confusion. At 63, already frightened by the prospect of memory loss, Dorothy wrote a long journal entry specifying her children’s birthdays, her license plate number and her brand of hair conditioner, just so that she could hang onto those things.Having watched her father die of a brain tumor and her mother fade into an Alzheimer’s fog, Ms. Keaton worries about her own future and asks, “When does ‘Where did I put my keys?’ become a diagnosis?” She is realistically fearful. But the delight she takes in her son and daughter helps her keep those demons at bay.Woody Allen, who began letters to Ms. Keaton with pleasantries like “Beet Head” and “Greetings Worm,” once wrote her these fateful words: “I have decided to let your family make me rich! It turns out they are wonderful material for a film. A quite serious one, although one of the three sisters is a fool and a clown. (I think you can guess which one, ducky!)” That was the seed for “Annie Hall,” with a cast of characters not so different from the relatives this book describes.“Annie Hall” was full of WASP caricatures and silly exaggerations, she says. Nevertheless, descriptions of her brother Randy recall Christopher Walken’s hilariously unhinged presence in that film. As for Grammy Hall, who glared so poisonously at Mr. Allen’s character, Ms. Keaton recalls her real Grammy Hall as saying that Mr. Allen was too funny-looking to get away with some of his gags, “but you can’t hurt a Jew, can you?”“Then Again” includes Dorothy’s response to an “Annie Hall” screening, and her pride in her daughter’s success. It also points out that Ms. Keaton was involved with Warren Beatty when the film came out, and contrasts his way of wooing her with Mr. Allen’s. Mr. Allen adored and babied her. Mr. Beatty could be brusque. (“You’re a movie star. That’s what you wanted. You got it. Now deal with it.”) But he could also wow her with wild acts of generosity that played right into her Daddy issues. Still, she says, “I wanted to be Warren Beatty, not love him.”Ms. Keaton also uses a third ex, Al Pacino, to illustrate how and why she had such difficulty making permanent connections. “He liked plain,” she writes. “I hate to say it, but I was not plain.” She has three “Godfather” films with Mr. Pacino to recall, each finding them at a different stage of semi-estrangement.Between its many glimpses of Ms. Keaton at different ages, its telling of Dorothy’s story and its loving portrait of Dexter, Ms. Keaton’s teenage daughter, this book manages to present the full spectrum of women’s experiences, from babyhood to adolescence, youthful insecurity (and secret bulimia) to liberating adulthood to slow, lingering death. Its power as a collage has been greatly enhanced by tight, punchy editing of the fragments that Ms. Keaton variously writes or excerpts. Some of its stories are universal and painful, yet this book is not mired in melancholy. Instead it’s inspiring in its empathy, wisdom and self-knowledge.When Ms. Keaton won the Academy Award for “Annie Hall,” she was congratulated by Audrey Hepburn, a female icon she had long idolized. Hepburn told the new winner that the future would be hers. But what Ms. Keaton remembers best about that night is a different idea of the future: she saw Hepburn as an aging star, and sensed that stardom was a dead end. She did not expect a pedestal to hold her forever.