We've just admitted that we are true book hoarders and got ourselves some more Ikea Billy bookcases. Boring, I know, but we have some already and they are reasonably priced and not too bad looking. One day I'd love to have a whole room full of books (I think these rooms might be called libraries) with floor-to-ceiling bookcases. In which case, this bookcase by Danny Kuo would be most excellent:
It's a prototype at the moment, but what potential!