Time to Purge???

I'm coming to the realization that some of the books in this house just have to go. It's so hard, because I LOVE books. But, I'm running out of shelves. I think I need to institute the policy I used to have with buying other stuff - for every new thing purchased, two old things need to go. I have no problem getting rid of old clothes, knick-knacky things, unused kitchen appliances... So why not books? It fascinates me that books each hold some sort of knowledge between their covers. If I read a really good one from the library, I will sometimes even go buy a copy just to have for myself. I think I am trying to preserve that knowledge, or at least remind myself that I now have it. I generally try to get novels from the library, and have no problem not keeping a copy myself. But classics, non-fiction, and now of course, homeschool texts and history books are accumulating like mad around here. How do I choose which ones go and which ones stay? What should I do? Help!!!!!