Remember the Goosebumps Choose-Your-Own-Adventure books? You'd read them and at the end of every chapter you had to make choices: to go inside the creepy dark tunnel or not, to investigate the noise coming from downstairs or not, to split up from your best-friend to cover more ground at the haunted carnival or not. Each choice changed the story slightly and there were two or three different endings to the book, depending on the choices you made.
Sam Bowring has written a book in a similar format for ADULTS, and I think it's superb!
Butler to the Dark Lord is...