"REK is a bookcase that grows with your book collection," writes Reinier De Jong Design on its official website. "The more books the bigger the bookcase gets. The zigzag shaped parts slide in and out to accomodate books in the resulting voids. REK will always be full, regardless of the quantity of books. Also the books can be arranged according to their sizes. The narrow spaces are excellent for magazines."
Price is "on request", unfortunately, which suggests I'll be sticking with my cheapo Billies for the time being.