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.
We've used a similar corner shelving unit to create space in a small 7 by 9 ft room. It's a great, easy shelving solution for small spaces (although installing it was a bit tricky as the walls weren't plumb.
I see a lot of "almost" bookshelves. They hold books and look "design-y" but they waste energy and space. If you are really into books, it's not about the cute shelves. It's about the books. Make that space hold as many as it can.