I would rate this about a 3.7/5, but I can't do that, so I'll round it up to a 4/5.
I have low expectations when it comes to erotica. They tend to have little story and are essentially just a repetitive string of sex scenes. This one actually surprised me with how good it was. There were solid story and background, so it felt like reading an actual novel. The sex didn't seem pointless because there was a scientific explanation as well as an instinctual one.
If I had to complain about something, it would be the, "I love you," section of the book. It didn't seem real. I can buy Merinus loving Callan because of when she said she started loving him, but I can't buy Callan truly loving her as much. In the animal sense of loving her, yes, but humanly, no. I'm a bit skeptical on that sort of thing when it doesn't happen until after regular sex.