It depends on a lot of things like some of you have said. I used to keep all my books because I thought I'll need them some day. And I really did need some of those books. But the truth is that most books became useless and now they are just taking precious space.
But again, it depends on the book, the subject, and much more.
And of course, nowadays you can find almost any information on Google, Wikipedia, HubPages :). Personally, I enjoy reading paper rather than my LCD screen.