A book job is, quite simply, fellatio in which one member of the act is also reading a book. A book job is also known as literahead.
Person 1 - "Awww man, I can't wait for tonight, my girl's gonna be giving me a book job."
Person 2 - "Sweet, what are you gonna be reading?"
Person 1 - "I thought parts of 1984 would be appropriate."