Planet Algol has an interesting post on unique spellbooks HERE. I really, really like this idea. It's a fairly easy way to add flavor to your campaign setting, and even little tidbits of history. And the possibilities for adventure seeds are pretty much included with any given spellbook. A spellbook written and used by an evil lich instantly provides a general theme for the spells within, a monster that must be defeated, stories of heroes who fought the lich and lost, and a cool description such as the book is encased in bone and the words are carved into the smaller bones that serve as the pages of the book.
Unique spellbooks have a fairly long history in fantasy and even D&D. More than a few articles were written about them in Dragon magazine. They also provide the DM with ways to include variant spellbooks. A massive tome wrapped in demonskin is pretty cool. A set of wind chimes that whisper the spells into your mind when exposed to a breeze at midnight is even cooler. And the chimes spellbook is something that the players will remember years later, long after they've forgotten much of the actual campaign.