That's easy - fiction? The longest fiction book I've read is probably "Jean Christophe" and I can tell you in a sentence exactly what it was about (all 2000 odd pages). It's about what to do when, in the process of trying to make something new, you look deeply into your society and see the lies in it, and realize that ALL societies are this way - how to handle disillusionment and not become sterile?

Scott(s) told me exactly what the book was about in a much shorter sentence - a synthetic mythology.

The question then becomes - is it a unique one or a repackaging of existing one? A new mythology would certainly develop a large following.