I think this book is absolutely brilliant, but I don't think I care for Wolfe's writing, which makes this difficult for me to rate. If someone whose style I liked better had written it, I might have given it 5 stars, because I do think it's fantastic.
I can't go too much into the plot, because I do think this book is best read naive.
Basically, two (twin) planets, Sainte Croix and Sainte Ann have been colonized by humans a long, long time ago. There are mysterious aborigines that may or may not have existed (on Ste Ann). If they did, they were not human but could shape-shift to look and act like humans.
The book is composed of three stories that pull elements of this twin system, the societies of both planets, questionable aborigines and anthropological study together in interesting ways. There are interesting themes delved into - nature vs nurture (inheritability of traits), evolutionary theories, post colonialism, etc.
It's good. It's very interesting. I just don't think I'm a fan of Wolfe's style.