There's no way for me to explain the rating without spoiling the book. I normally wouldn't bother, but I waited a week and I'm still angry at this author, so I'm going to hide my spoiler and spew some venom.
This book starts nicely. It's not earth shattering, but seems like a good mysterious fantasy for the reader to invest in, and it continues that way for 90% of the book. She has two equally interesting and very different lives. It all turned on "the butterfly effect" or one small thing that happened or didn't happen which took her life in wildly divergent directions. I loved the book for 90 percent or more of it. It was intriguing and interesting. The characters were lovely and her confusion palpable.
We don't know if these are dual lives or how those dual lives would work out. We're led to believe at first that the bookseller is one helluva dreamer, though it quickly becomes clear that's not the case. I was interested to see how this would all shake itself out, and I had high hopes for realistic fantasy based in a time not that far gone where we could understand the cultural norms without a lot of work on anyone's part: author or reader. I didn't think it would be Pulitzer great, but an interesting read for sure.
It lost me completely at the end. All of the good will she'd built up with me went through the floor. I got genuinely irked at this author. We went from a good fantastic realism/dual lives to a story of bizarre mental illness(? maybe -- it's just not explained or discussed. We're supposed to believe the whole thing is just the way it is.) It all happens very quickly and without any explanation or any of the reasonable steps that might have been employed if any of it had actually happened.
So I don't know what to call this book genre-wise, but fantasy it ain't. If "bad" was a genre, that would work. Mostly, it's an author I won't trust enough to give another chance.