This is a wonderful book! I gobbled it up in one day and was crying by the end (a good indicator of how it affected me).
It won the Nebula Award in 1966 which is how it got onto my reading radar (I am working on reading the Nebula winners this year), and it truly did merit the award. Being more realistic fiction than science fiction because even though science is the catalyst for the story, it is told in progress reports written by the protagonist and they were riveting reads. I can't really say more than that as I would hate to give away the plot, so just go and get it and read it ok?