IN HER NEW MEMOIR Ongoingness, Sarah Manguso traces the beginning of her diary to one moment. "At an art opening in the late '80s," she writes, "I held a plastic cup of wine and stood in front of a painting next to a friend I loved. It was all too much." That night, overwhelmed by the fullness of life, she started the diary she would keep for 25 years—a diary that would become an almost debilitating obsession, a futile attempt to document every moment.

Written nightly in the present tense, then later edited and revised, the diary built up over the decades into an 800,000-word tome. "I couldn't face the end of a day without a record of everything that had ever happened," Manguso writes. The idea of life without it made her extremely anxious, made her life seem meaningless, her existence feel fleeting. "Perhaps all anxiety," she writes, "might derive from a fixation on moments—an inability to accept life as ongoing."

But things changed when, late in her thirties, Manguso became pregnant. Her memory suddenly turned poor, her perspective on life shifted, and after her son's birth, her priorities became focused on him. "He needed me more than I needed to write about him," she writes.

Manguso deals in these larger truths: A story about an excessively long diary and the birth of a first child is a way into ideas about memory, the drive to write, aging, loss, care-taking, and mortality. She conveys these ideas through prose that is most often referred to as "spare." Her concise, compact sentences have clearly been whittled with a poet's eye—each word carefully chosen, no room for frills. The effect is both cold and revelatory. Nearly every page of Ongoingness has a line that knocks the wind out of you a little (and most pages hold only a short paragraph or two). Her sentences demand to be read aloud, or quoted and applied to other contexts.

By the end of the book, having left behind the desire to compulsively document each day, Manguso is less anxious. She writes with a peace that's almost eerie, one that sometimes ventures into oddly morbid territory. "My goal now is to forget it all so that I'm clean for death," she writes at one point.

Ongoingness is a very short book about a very long diary that we as readers never get to see any part of. The meditations on the diary cover a wide range of emotional reactions, and, in this way, Manguso offers a variety of possibilities as to what the diary could stand for—a symbol of impermanence or obsession, a tribute to the taxing but exciting stage of early adulthood, or a call for active engagement. "The forgotten moments are the price of continued participation in life," she reminds us. It's this open-ended speculating that makes Ongoingness so compelling, and makes it linger for long after it's over.