Crate and Barrel have just released a new modular system of shelving which they're calling "Checkers." This is the latest addition to their recently launched "ready to assemble" collection. Checkers is basically an easy to customize program of stackable shelves, that come in 2, 3, or 5 unit cubes. They also have alternating open and closed off backs. What I really love are the more substantial looking, clean lined, metal base units that come in single, double or triple cube widths. These could be great for all kinds of solutions.
Aside from just a bookcase, I can also see them as a great room divider, or turn them sideways, and get a nice set of night stands, or even a long entertainment unit. You can just keep adding as needed. It's only available in black right now, but hopefully they'll release it in other colorways soon.