Here are the labels that I use to organize my classroom library. I'm a little wacky in the way I sort my books--some are done by general categories (Animal Books, Class Favorites); some are done by genres (Fairy Tales); some are by character (Maisy Books, Clifford Books) and some are done by author (Mo Willems, Kevin Henkes, etc.) It works for us. I want the kids to be able to easily find books and then put them away when they're finished!
I use the tried and true colored dot sticker system to label the books. When I run out of colors, I add a colored star to the middle. For example, all Mo Willems books have a black circle with a green star in the middle. Between that and the labels on the bins, the kids do a pretty awesome job of keeping the library neat.
You can also see I have leveled books. Those are die-cut numbers on the blue and green tubs. I use Scholastic Book Wizard to help me level books. I do a lot of "educated guessing" on books that aren't in their system.
I put themed books on the leaning book stand. It's where I put most of my non-fiction books--but I only pull them for specific units (i.e. these are "Back to School" books). This year, I plan on having a more permenant expository book section. I just have to find a place to put it! So, stay tuned...