I looooove the Ikea Expedit bookshelves; they're sturdy enough to take a lot of weight, and if you use it as a room divider (rather than against the wall) you can store paperbacks on both sides (which enables easy access to all books while allowing double-deep storage).

Some lovely hacks, courtesy of ikeahacker:

http://ikeahacker.blogspot.com/2009/07/expedit-storage-and-room-divider-from.html

http://ikeahacker.blogspot.com/2007/01/expedit-grande.html