The thing with being a renter is that the place you're in is so small (but the location! the cheap rent! your landlord's great!) that you're constantly fiddling with the layout. And then, of course, you want something cool. But if you're going to plunk down some money on something it's gotta work in a number of situations (like how will it look when you move into that loft downtown you've been dreaming of? Or at least into a place with a separate bedroom?) Consider the Swedese Libri Bookcases.
Yes, they look like those standard "lean against the wall" bookcases but they're not. Designed by Michael Bihain, they've got a flexibility those bookcases lack. Their adjustable legs means that they can hold your books straight even if your floor's slanted (so that's what they meant by "Old World Charm"). Use one set. Use them in multiples. Lean them back to back to form a room divider. And throw in the fact that they don't need anything to attach them to the wall. So that when you do move, you can get your deposit back. And get another set. Click here for more info.