In my opinion, there is nothing sadder than an empty bookcase (ok, I can think of a few things sadder). I've nearly filled all my current shelf space and taken to stacking books about the house in various piles in random locations until I have enough to fill at least half a new bookcase. The REK by Reinier de Jong is the perfect solution to such a predicament. "Book shelves rarely adapt to your collection of books and periodicals but here is one that does." The shelf is adjustable and provides compartments for all book sizes to insure that it always appears full. When you get more books, you just expend it. One word, brilliant!
(Found via Furniture Fashion)