I can pick out two themes here. As this is alternate history - and really, at its very, very best - the idea that small actions create reactions that can change the world. We all know "for want of a nail, the horse was lost" but the linchpin, Pat/Tricia's yes/no to a marriage proposal - is so completely unrelated to war or peace. A better cliche is the idea expressed in Jurassic Park: "A butterfly can flap its wings in Peking and in Central Park you get rain instead of sunshine." Or quite possibly It's A Wonderful Life: "One man's life touches so many other lives." In ways we can't even imagine. That's terrifying and exhilarating.
The alternate timelines - two of them actually, both familiar yet distinctly different from our own - are really just a science fiction-y backdrop to the strong and interesting lives of Tricia/Pat. In one timeline, Tricia says yes and marries a boorish beast; in the other timeline, Pat says no and forms a loving relationship with another woman. Using these strong women's lives and experiences, she's able to subtly highlight what's happening in each alternate timeline that's different from our own (nuclear exchanges, world wars, etc.) as well as some cautions and insight on our own timeline as well. It's interesting that the abused Tricia ends up in a more peaceful timeline; strong Pat who superficially has an easier, more satisfying life ends up in a far scarier world.
Walton obviously loves her alternate history - see the masterful Farthing - but she also strongly writes about sexuality as well. Her lesbian characters, at least to me, seemed incredibly and vividly real; her gay characters in the Farthing series, tragic as they were, felt the same way.
My rating: 5 of 5 stars
Alternate history at its very best , and more than that. This isn't your typical science fiction time travel book (but then Jo Walton books are never, ever typically anything other than brilliant); rather, Walton uses two alternate timelines to tell the story of two women, who are the same woman, and how the answer "now" or "never" to a wedding proposal changes not only them but the world. She uses this woman, Pat/Tricia, to compare and contrast; she is the shadow puppet against the scene changes of vibrant and changing histories (that are different from our own). Because of the small incidents in this woman's life (lives), the bigger incidents taking place in the world around them (nuclear war, political upheaval, etc.) loom on the backdrop behind her. Walton is an incredible writer of real characters and intricate, beautiful plots; there is plenty of attention to detail (she always reminds me of a Connie Willis who writes shorter books). There is a delightful feminist maxim that runs through this book that I loved; I also thought she wrote strongly and honestly about sexuality.