How cool is the Tree Bookcase by Roberto Corazza? This single-cast plastic bookcase is open on both sides allowing it to be attached to a wall or standing freely, leaving access to the books stored on the "branches" of the tree.
Turn your books into works of art using the Invisible Book Shelf - This is so awesome, I have so many books and I love them all, but storing them on a bookshelf doesn't show how great they are individually.
Small Space Secrets: Swap Your Bookcases for Wall Mounted Shelving. Here's a creative shelving arrangement that uses a deeper shelf at bottom to create a sort of bench. The bench can serve as extra seating for parties, or as a display space.
I wrote a whole post about swapping your bookcases for wall-mounted shelving. Wall-mounted shelving takes up less space, and can almost appear to blend in with the wall. Image from Stadshem, via Freshhome.