I've always loved the Bruno Rainaldi Sapien Bookcase ($198-298) — and its impersonators. The design isn't particularly innovative, but somehow books just look more fresh to me stored vertically. It's space-saving, too.
I began stacking my books about a year ago I guess, and I've ended up with a very, very tall stack that certainly gives the Sapien a run for its money. Lately, picking a new read is a bit like playing a game of Jenga. But it's worth it because I love the way it looks, and it seems to baffle my guests.
There is a science to it, though. The stack wouldn't stand were it not for the very hefty book my sister gave me at the base, Phaidon's 30,000 Years of Art — thanks! You must stack them from the bottom up descending in width. But, if any book is wide but not very deep (i.e. a Zagat Guide), it should be saved for the top. If you have a wide book that is deep, but is very thin, you should either stack it between two thicker books of similar widths and depths, or just don't stack it at all. It's all hard to grasp until you've played around with your volumes, but it'll make sense once you do.