The Vampire's Assistant
The main character of the book wants to save his friend. So he goes to a vampire that he seen at the circus de freaks, that came to town. The vampire offers to help him, but only if he does him a favor in return. That is to become his assistant. At first the main character refuses but in the end has to save his friend.
After he saves his friend, he then decides he has no choice but to go with the vampire and have his family think he is dead....