This was one of the most enjoyable books I read recently. This is definitely character-driven fiction, and not just primary, but secondary and tertiary characters too were developed with unparalleled depth and nuance.
The plot revolves around a young woman named Rosemary, who is trying to figure out what happened to her "sister", a chimpanzee named Fern. In their early childhood Rosemary and Fern (who were born just months apart) were raised as siblings; Rosemary's parents treated the chimpanzee as their own child, encouraging her to do everything Rosemary did. Then, when Rosemary was five years old, Fern suddenly disappeared from the household, and Rosemary never fully recovered from the loss of her sister.
On one hand, it wasn't hard to figure out the mystery of what happened to Fern; it was clear that Rosemary's parents gave Fern away because they just couldn't keep a chimpanzee at home anymore as Fern grew bigger and stronger. Historically, cases of raising a chimp as a human never ended well, because humans were never able to control the chimp's aggressive tendencies; this novel leaves no illusions that this attempt could have been anything else but doomed, and the adult Rosemary understand it very well.
However, there is a twist at the end that makes it particularly ironic -- but by the time it is delivered, we readers are quite skeptical whether we should believe it. That's what makes this book so captivating. As Rosemary tries to piece her past into a coherent narrative, it becomes increasingly clear that none of the characters' versions of events can ever be trusted. Rosemary herself doesn't trust her memory, believing that many of her vividly remembered childhood episodes never could have happened. But we also find out that throughout Rosemary's childhood her parents and older brother Lowell manipulated her, feeding her various lies, fictions, and non-answers to avoid accept responsibility for their actions. So later in the book, when Lowell delivers a key "revelation" to the now-adult Rosemary, there is no reason to think that he isn't manipulating her even then.
This book reveals, in an understated way (because Rosemary is never bitter or angry towards her family) how even highly functional, seemingly caring parents can be subtly cruel towards their children. They raised Fern among humans, knowing that she won't be able to live with them indefinitely, yet making it very hard for her to adapt to a life among chimpanzees. It was just as bad that Rosemary's father, a psychologist, treated not just Fern but Rosemary too as an experiment. While she and Fern were together, they were both studied by graduate students in her father's lab. Everything Rosemary said was interesting to them, but only because she was part of the human-chimp speech acquisition experiment. With Fern was gone, nothing Rosemary said interested them anymore, and her endless chatter became a nuisance.
What I liked best about this book was endless observations about the nature and unreliability of memory, about theory of mind, about animal rights, and the way those meditations were wrapped into suspenseful plot arc.