When is the last time you went in an culled your book collection? Not only will you feel good about decluttering, you will also do something green, because you can always donate books. Most libraries have a book donation area. These will either stay at the local library or go to underprivileged areas.
If you can’t stand the idea of losing a book, you can also do a swap! Get some of your nerd friends together and trade off! Set up a meet up and off you go.