First, and most economical, is Ikea's Expedit bookshelf (pictured left in the walnut finish). My friend has this bookshelf in black and currently is using it as a room divider. Ikea's website claims that it must be attached to the wall at some point, but my friend says that it is sturdy even on carpet. The bookshelf is deep enough so that my friend can set it up with books on both sides of it such that there are two rows of books in each cubby, which, obviously, enhances its desirability as a room divider. The bookshelf comes in black, white, and a light tan wood color in addition to the walnut. The pictured version is $199; find it here. Ikea also makes the bookshelf in smaller versions, such as this slightly smaller version ($159) and this much smaller version ($89.99). As with most Ikea products, the Expedit collection pairs with other storage items, such as bins and drawers for the individual cubbies, and attachable desk units.
Second, on the less economical side, is Generate's Quad bookshelf (pictured right). This bookshelf also can function as a room divider, although the areas where there is "interior" might not look as nice on the other side. I think the orientation of the cubbies is this bookshelf's best feature. It comes in the pictured white with lime interior, black with lime interior, black with white interior, white with black interior, ash brown with white interior, and walnut with interior. The pictured version is $999; find it here. The bookshelf is also available in two other configurations which are both an extra $750.
Third, continuing on the less economical side, is Smart Furniture's customizable bookshelves. I have pictured (left) the six foot wide staircase bookshelf. Smart Furniture allows customers to mix and match bookshelf parts to create the perfect storage unit for your space. The pictured staircase bookshelf is $530; it comes in the pictured black, cherry, espresso, maple, pearwood, and white; find it here.
I also like Smart Furniture's five foot wide pyramid bookshelf (pictured right), which, if you have the space, I think would look great beside a larger bookshelf used as a room divider, perhaps positioned at a bit of an angle. It would also look great in the corner of a room set a bit out from the wall, again on an angle as to the wall. It comes in the above colors which will allow easy matching with other furniture. The pyramid bookshelf is $249; find it here.
Finally, on the least economical side, is Brave Space's Stagger bookshelf (pictured left). Obviously, the best feature of this bookshelf is the asymmetrical staggered shelving. It also is great as a room divider. Based on the dimensions, I think that, like the Ikea bookshelf, this bookshelf is deep enough to double stack books so that a row of books can face out on either side if the bookshelf is used as a room divider. Also, the bookshelf is made from bamboo; very cool. It is a completely unaffordable $3,975; find it here.