I continue to kick myself for not discovering Theodore Sturgeon earlier. This book is not on a topic I'd normally have considered picking up, but I found it well written and engaging throughout. The SF element is technically crucial to the concept, but in fact it's not essential to the story, which is about people and relationships.
Sturgeon lets the story down by throwing in large masses of exposition at the end - needed to explain the mechanism and mysteries of the story, but suggesting that he trapped himself at the end of the story, and didn't feel like rewriting the entire book.
Nonetheless, this is a good and enjoyable book about character and humanity. I recommend it.