I consider a great book one which causes my mind to think, period. The more the book causes me to think the less I want to put the book down. The degree to which a book catalyses mental energy is the degree to which I it consider great.
Lectures, dance, movie, sermons, photos, professors, all share the following judgement method in my thinking. If they cause me to think, especially if they cause me to think in line/order about a coherent systems.
Fantasy and sci fi are of interest to me. When written, well sci fi and fantasy developes a picture from which a perspective, new to the reader, can be investigated by the reader.
As I am trying to understand my thoughts, my conscious, my subconscious I look at liturature for aspects to those related systems. When a writer like Robert Jordan, Stephen King, William Shakespeare, John Milton, Homer, and others who have considered a new world view/structure they impart more than words. When done well a good story etc. makes it easier to analyze both whether characters fit into the story in a coherent manner and if they do, how do their personalities interact.
On the other side a writer can have the same effect on the reader in a writing nonfictional biography, academic text, historical fiction and informational/self help books. When books propel me to eureka, aha, and enlightenment, I consider it is a Great Book. Even when there are components of thought I do not agree with throughout the book.