But unusual doesn't have to mean weird or standing out because it's so totally strange. For today's purposes unusual means something rare or out of the ordinary.
When a kid hides out under his bed covers with a flashlight just to finish a book, I would say there's something unusual about that book. My son did this once a long time ago, and I pretended not to notice the flashlight because I thought it was completely awesome that he thought he should be sleeping but that he had to stay up reading!
In the time since, we've discussed that if a book is that good, you may read as long as you like--but by no means may you be crabby the following day.
So what kinds of things make today's books unusual? There are so many choices, so many great books out there, what is unusual enough to keep a kid reading long after they should have turned out the light?