Sven doesn't like it much because he left Andor when he realized Andor would never love him and would never think of him as more then just a part of the herd. He thinks this because as Sven grew older Andor started to withdraw from him. Also, Andor not feeding from Sven or making the moves on him made Sven feel like he wasn't wanted.
Now, Sven's part of the 'foundation family' which is just a bloodline that's served Andor since Andor turned into a vampire, which was a long, long time ago because Andor was a Viking. The foundation family is considered the elite, and most important, of the herd.
When Andor was a Viking, though, he loved a woman named Elessa but when he was turned she refused to stay with him and become a vampire herself. She had a family but she still served Andor. I was kind of confused on this part because I think she refused to have sex with him after he turned but still gave him blood. Although it could have been that she completely refused to help him at all but either way, Elessa died in Andor's arms and made him promise that Andor would always treat Elessa's descendants as more then just sheep. She made him promise that Andor wouldn't touch them either unless they asked him too.
Which was the whole reason for Andor withdrawing from Sven because he made the promise to Elessa all those years ago and refused to break it by taking from Sven without Sven giving permission first. But when Andor realized he was going blind, he needed a pure blood source and Sven was it.
There's not much that happens in the book except for at the end. Up until then, it's just Sven and Andor going back and forth with misunderstandings. Sven feels like Andor just wants him for the blood. Andor thinks that Sven knows that's not true. To me it didn't seem like it was too much (of the misunderstanding) but it might to some people.
Andor has a doctor come by and find out what's wrong with him and how he was poisoned and how the herd was poisoned. The doctor draws blood from everyone and finds out that the only two people that weren't contaminated with silver (I guess that's a no no for vampires, lol) was Sven and Josiah (even though Josiah was filled with drugs and alcohol)
To me, the best part was the end where Josiah, Sven's cousin, basically kidnaps Sven and takes him to another vampire who wants to take over Andor's territory by thinking that he could torture Sven and Andor would come running, which he does, but the other vampire, (Castillo, I think) believes that Andor would be an easy kill (since he's had Josiah poisoning Andor's herd, thus making Andor weaker not only in eyesight but also in healing.)
But, Andor proves that he's not that weak and kills Castillo. Sven gets tortured a little. There's not much on that besides a little use of fire and some cuts but Andor saves Sven.
They find out that Josiah did what he did because he was jealous of Andor not using him as a donor. He was also jealous that Sven was the favored one and not him. So, he went to a vampire that promised to give him what Josiah wanted.
Anyways, that's about it. Of course, Andor and Sven work out their problems and Sven realizes that Andor does love him.