One thing book bloggers like to talk about in reviews is the so called un-put-down-ability of a given book. But really, what makes a book so good that you can't stop reading? It's different, of course, for each person, but here is a list of what makes something engrossing to me.
This is the obvious one, I suppose, but not something to be overlooked while writing. It's natural to have some variation within the speed of a book - scenes with high action and then scenes with, perhaps, more emotional involvement.
However, it's still necessary that there is enough uncertainty and worry generated that the reader has to - no, needs to - know what's going to happen next, and that is what will keep the pages turning.
Not all books have elements of humor, but one of the surest ways to keep a person interested in reading more is funny narration or events or both. People in general like to laugh (I wouldn't want to meet someone who doesn't), and a funny book will keep them entertained, simple as that.
This applies to more than just fantasy. A writer needs to craft a world that can completely suck in a reader and engross them in that place - and therefore the story.
So, what do you think makes a book un-put-down-able?