I read this when it first came out; please correct and forgive inaccuracies of memory. (Appropriate to the story!)
Patricia, an Alzheimer's patient, is in a nursing home. The nurses think that she recalls living two completely different lives (and is slipping between realities now) because she has dementia; we, the readers, know that she's recalling alternate timelines.
In 1949, she agreed to a marriage proposal, or not. The woman who agreed became Trish, trapped in a miserably abusive marriage... but also living in the best possible world as far as the general good is concerned, with peace, prosperity, and a moon base. The woman who declined became Pat, who falls in love with a woman, travels, and has a life full of love and self-fulfillment... in a world that slides into nightmarish total war, and seems to headed straight for Armageddon.
Though there are plenty of full scenes with dialogue and so forth, there's also a lot of summary narration. This works surprisingly well; my interest only flagged in the last fifth or so, when I started losing track of the multiplicity of alternate children and grandchildren and their significant others. It's a book about two largely mundane lives that inexplicably has the narrative grip of a thriller. I credit Walton's writing skill for this, and I'm still not sure how she did it. Between the depressingness and the summarizing, by all rights I should have bounced off this book rather than reading it in a day.
I didn't write about the book till now because I had such mixed feelings about it. Artistically, it's very well-done - an unusual use of tell-not-show that succeeds in (mostly) being compelling reading. However, I also found it excruciatingly depressing. It deals centrally with five of my top ten most depressing subjects: Alzheimer's disease, agonizing death by cancer, nuclear war, domestic violence and emotional abuse, and being consigned in a nursing home where you're helpless and mistreated and cut off from everything that makes life bearable.
Regarding the alternate timelines, the ending strongly implied that it was Patricia's choice of who to marry that led to sweeping changes between the timelines. I assume it was a "butterfly effect" in which she made one small change that led to several other small changes that ended up having a gigantic domino effect, but I would have liked to be able to see some of how that happened. I couldn't figure out what it was she did that was important. If I recall correctly, history started changing in big ways right after she either got married or didn't. Trish did get involved in political volunteering, but if I recall correctly, history had already changed at that point. Am I misremembering when history started to change, and it was the volunteering after all? Or was there some other crucial action that I missed?