The New York Times Room for Debate blog has a post on how to sort your books to keep the wheat and sell the chaff. It's tied to the changeover of the year but, man, this strikes me as the most reliably groovy way to pass a few hours regardless of the season.
Whenever my wife gets into a Mike-should-do-some-organizing phase I offer to take responsibility for the bookshelves, and that means many happy hours of checking dog-eared pages for the good stuff, and testing spines for springiness (a split spine doesn't damn it but does move it closer to the sell pile), and sorting books by subject (Fitzroy Maclean's Eastern Approaches has probably never left the side of Peter Hopkirk's The Great Game: The Struggle for Empire in Central Asia in the ten years since I've read them).
I did a major sell this summer, and it felt good at the time; but I discovered long ago that I have a pretty terrible instinct for what I won't miss. I sell it, and then a few weeks later I go to look something up in it. I'm still grieving for the Historical Atlas of Central Europe I sold then, and even as I stood at the sell counter at Half-Price Books I knew I'd regret the loss of the NYRB edition of Burton's Anatomy of Melancholy with the freaky blue-green spine. But it had to go: its blue-greenness was so haunting in its beauty that I could no longer have it in the house.
In any case, the NYT post has some good culling advice from the likes of Francine Prose and Jane Smiley and is worth a read.