Strahov Library, Prague
Now, I'm guessing most of you don't have a "Beauty & the Beast" castle style library (and if you do, please invite me over for a week of perusing!). But you probably have a bookcase or 5 with reading materials collected over a lifetime of reading.
The challenge: decide what you'll really read again and/or want to keep for some kind of sentimental value. I recently rid my bookcase of a few books from college that I had never actually read. They were great philosophy type books and I wanted to be the type of person who would read them for pleasure. But I had to own up to the fact that I am not.
Now, in full disclosure, I still haven't rid my bookcase of my advanced mathematics text books from undergrad. While I will very likely never have a need for them again, they are such a part of who I am that I cannot yet bring myself to get rid of them. However, I have limited the space I allocated for books and only keep the amount that will fit in that space. So if you have some books you don't really need anymore, don't worry... just make sure they are given the proper amount of space and not taking over your entire room.