How does a person maintain interest in a book so as to read much of it in one sitting?

I've managed to read some books in one sitting but then again, they're rarely 'hard going' books but more like, easy reading fiction and things like Twilight :p I find the same like you for books I have to read but then I just keep working through them, telling myself that in this hour I will read it and try and get to a certain page number. If its boring I work on chapters/page numbers and getting over them. If it's interesting to me then I don't even remember those techniques.