The thing I love most in fiction is when it gives me something that is real outside of the fiction.
A turn of phrase, a concept, a new way of looking at the world that shakes up the insides of my skull a bit like a snow globe. Anything real in the middle of a fantasy, a truth in the middle of a fairy tale of escapism. That contrast, the shining gem in a plain box, I love it.
Some books are all truth. Some books are all fluff. Those have their place, but I love the book most that's a few sharp needles hidden in the straw.
I do not love a book that is a trick, though, or one that feels superior, like it's doing me a favor by dressing up the hard thing in a pretty dress. It - and this is a fine line, a matter of perspective - it has to be done with compassion, with understanding, with knowledge and awareness. It can't be written as dismissive of disdainful of the person who loves it.
That's all.
