An identity crisis is part of many American lives. But whether the quest for self comes at 14 or 40, the average person can depend upon a few unshakable facts, like her name and the names of her siblings. Sarah Saffian assumed she knew these basics about herself. Adopted at birth, the daughter of Marvin and Kathy Saffian, sister to Max and Rachel, Saffian planned to research her origins someday, but hadn't got around to it before the telephone rang one morning when she was 23. ''Sarah, my name is Hannah Morgan,'' the caller said. ''I think I'm your birth mother.''

That happens on the first page of ''Ithaka,'' a multilayered memoir that probes profound questions of identity. This is not a typical story of a birth mother and child reuniting if such a story exists. For openers, most adoptees choose to find their birth parents, not the other way around. The difference meant that Saffian had no control over whether she wished to make contact; it was done for her.

In the space of a few minutes, Saffian learned that not only her birth mother but an entire biological family had found her. Her birth parents, Hannah Morgan and Adam Leyder, were unmarried when they gave Saffian up for adoption. But they later married and bore three additional children, Saffian's full siblings. Saffian responded to this stunning news with some of her own: Her adoptive mother, Nancy, had died when Saffian was only 6. Kathy, her stepmother, had filled Nancy's place.

Saffian explores this complicated material in beautifully nuanced prose to create a book that grows richer page by page. It chronicles a three-year correspondence with her birth parents, as Saffian tries to decide if she will ever be ready to meet them or even talk with them again by phone. The story shifts from present to past and back, giving readers a sense of the tension Saffian felt as she struggled to place her birth family in her world — indeed, as she tries to discover where her world might be.

Through letters she learns the most intimate details of her birth parents' early lives, including their disagreements about whether to keep their child. Perhaps because of Leyder's own traumatic background — his parents died when he was very young, leaving him to be raised by family friends — he was much more overbearing than his wife in pressing Saffian for a reunion.

Both birth parents talked about how much they love and take pride in Saffian, sentiments she appreciated but questioned. How could these strangers say they love her when they last saw her as an infant and have little knowledge of the person she is now? At the same time, she wondered how they have influenced her through the unbreakable bond of biology. Her personality traits, her interests, her way of approaching the world — how much of that is genetic?

As Saffian's sense of identity splintered, even her name became open to question. She discovered that in a surreal quirk of bureaucracy she exists only under her birth name, Susan Morgan, in the Birth Record at the New York Public Library. At the Bureau of Vital Statistics, where birth certificates are kept, she exists only as Sarah Saffian.

An unusual story hardly insures a great memoir. But Saffian selects language with finesse, creating images that deepen the story. Her book's strong rhythm and pacing, particularly its provocative chapter endings, keep the reader engaged. The only section that falters is ''Origin,'' Saffian's embarrassing fantasy of herself in the womb. Even in a personal account, there is such a thing as too much intimacy.

One could also question her brashness in comparing herself to Odysseus, who took 10 years to reach his homeland, Ithaca, following the Trojan War. Given the complexities of her story, the allusion is reasonable if a bit excessive. Saffian succeeds in making the reader care about her journey, and her conclusion suggests she reached where she hoped to go. It is surely no accident that the last word of the text is ''whole.''