when you are breeding for certain traits in animals you do what is called line breeding. If it is a recessive trait you keep breeding very closely related animals that have the desired trait until you get all or most of the offspring with that trait. Which is one of the reasons so many pure bred dog breeds are so screwed up. Your dalmatian has perfect spots but it also has bad eyes and a weak heart. So maybe they need to inbreed to keep the super natural thing going.

More likely though it is simply that people who read these books like freaky sex stories.