In her new book, "A Song at Twilight" (Blueroad Press, 304 pages, $18), Minnesota poet Nancy Paddock has written an intensely personal and heart-shattering memoir about being a caretaker for her parents as they descend into the murky landscapes of Alzheimer's and depression. It's also a fascinating, illuminating meditation on the indeterminacy of memory and memoir, a struggle to remember despite the diminishment of the mind.

Her mother's Alzheimer's creates a recurring inversion of roles. Paddock remembers having "imaginary friends" as a child; her mother never once punctured the illusion. Now it is Paddock's mother whose mind is filled with fantasies, visions and imaginary events. Faced with one glaring fabrication, Paddock hesitates to correct her mother: "Even if her story has no meaning, why do I, who once played with angels, need to correct her?"

Lyrically powerful, Paddock's narrative moves from the darkening present, as her parents, Lois and Ralph, struggle with their mental capacity, to the past -- their love affair and family life, as recorded in family letters, old photographs and stories. Like any loving daughter, Paddock struggles to balance her role as wife and career woman with the requirements of taking care of parents who need more help every day. There are good days and bad. Paddock seeks to give them the illusion that they remain independent, but she knows it's a mirage. Her parents are angry both at the loss of their capacities and the need to have their three daughters acting as caregivers.

Paddock's compassion fuels every page. Watching her mother's memory slowly disappear, she asks, "What must it be to feel your hands shake ... to botch recipes you knew by heart, to grope for the spelling of simple words ... to be taken everywhere like a child?" Paddock must ultimately accept her own inability to "fix" things, but she battles like a tiger to defend her parents' dignity.

There's plenty of dark humor here. As Paddock discusses with the nursing staff the possibility of using antidepressants, Ralph sardonically asks Lois, "Do you feel like a bug on a screen?" Later, one of Ralph's daughters asks him how he'll dress up for Halloween, and "[h]e grimaces, gestures toward his ravaged face. 'Isn't this scary enough?'"

Just as her parents descend into darkness, Paddock becomes depressed, too. She's brutally honest about her own ambivalent feelings. "[D]rugging them -- and myself -- also dulls my own anguish." Both parents die by memoir's end, within days of each other, and Paddock's memoir transforms into a determined attempt to remember, to commemorate what's been lost. "A Song at Twilight" is Paddock's beautiful, fragile and unforgettably open-hearted effort "[t]o write my way out of a labyrinth of sorrow," a sorrow that connects each of us in loss.

Chuck Leddy is a member of the National Book Critics Circle and also reviews books for the Boston Globe.