Here's an interesting article on how to purge:
I do take books that I know that I won't reread to my neighborhood used bookstore but I'm sure that I'll never be disciplined enough to pare down to 20 books that I haven't read. I need to have enough choices to suit my mood.
We still have a lot of books around here but I just took 3 BIG boxes of books to the library. I only kept books that either have semtimental value or that I might want to read again.