To me, a great book is one that you can get lost in. It's almost like, as you read it, you forget about everything going on in your life. So, by the time you are finished, you just sort of sit there for a while, processing what happened and reorienting to 'reality'. This is possible with all kinds of books, it's not limited to narratives, but it can be a rare and fleeting thing. So it's extra special when it comes along.