How incredible is this bright and airy library by Craft & Creativity on flickr? IKEA shelves and an incredible effort in organization and sorting. What an eye. Oh lord how I love it.
This version creates such a different mood with the dark wood and colorful accessories. Love the deep blue walls, too. Just stunning. This is precisely how I would want my library to look (possible in my dreams only, of course- le sigh).
bookshelf spectrum revisited originally uploaded by Chotda aka Santos
This one is more simplistic, and gives off a sort of bachelor-feel with the open shelving instead of bookcases. This would look terrific with inset shelves (the ones with no visible hardware that look like they're floating).
Rainbow Books originally uploaded by Andrew Coulter Enright
If you own a huge number of black and white rather than colored books, here is a way to balance the rainbow with the monochromatic to create a beautifully symmetrical look:

...And if you don't have enough books to fill the shelves, or prefer the spare, minimalistic look, try coordinating a few spines with other objects in their color family like this lovely display (yeah, I know, I said I didn't like vases in bookcases so much- I make an exception here.) Also, varying vertical and horizontal placement can create visual interest.

Have you thought about or actually accomplished organizing your books by color? Do you like this look or are you "over it" since "everyone's doing it"? I like to be a little different so it is strange to think about adopting something super-trendy, but sometimes trends stick because they are just plain awesome, so I'm calling this aesthetically pleasing idea wonderful enough to not care less whether everyone else likes it, too.