Holy buckets. What a fantastic read! This was one I definitely need to read again, now that everything (mostly) makes sense.
I am quite proud to say that I guessed the "twist" mere seconds (or should I say "words") before it was revealed. I wouldn't call it obvious, but there was only one plausible solution, only one way in which the story made sense.
I did have a very difficult time understanding Margaret as a character. Her suffering was not clear, and Setterfield began to truly lean on her pain towards the end of the novel. I preferred the Margaret at the beginning of the novel, who was fascinated with words and the many lives created by words. But I've never been in her position so relating to her would be difficult.
I definitely would recommend this book. Keep at it when you want to give up; I had a few of those moments myself. It's completely worth it.