I've owned a lot of books over the course of my life. This is what happens when you're an avid reader, a geek, the child of academics and have multiple advanced degrees. I've also moved many times over the course of my life, and I think everyone can agree that moving books sucks. They're heavy, they're bulky and they're an overall pain in the ass.
When I met D, a major fan of the public library system ('temples of freedom' is what he calls them), we had long discussions about the number of books I owned and was hauling around from place to place. Over the past four years I have gradually paired down the number of books I own, donating large quantities to libraries, schools, thrift stores, etc. Getting rid of books has been a difficult process, and it has given me some insight into the consumerism around book ownership. So here are a few of my thoughts on this issue...
I'm a bit of weirdo (as if most of you didn't know that already). There are some books that I have read dozens, if not hundreds, of times. I call these my 'sanity books;' they're the ones I pick up when I'm feel sad, lonely or just in need of comfort. These are books worth owning, but they only account for about 15-20 of all my books. So what about the others? A good number are books from college or medical school, which often represent intellectual pursuits that I no longer engage in (population genetics, sociology, clinical medicine, etc.), but I keep them 'cause I just know I'll get back into them eventually. Right? Wrong.
We all keep books from our past for various reasons (a few that came to mind: the emotional history they represent, hope that you'll go back to being the intellectual you thought you were in college/grad school, wanting to look smart in front of your friends (smart people have lots of books, right?), and so on and so forth). So why keep these books? They take up room on your shelf, you have to lug them around any time you move, and they need to be dusted periodically (given that I'm not the neatest person in the world, this last one is a particular issue. :)
And here's another reason to get rid of books. If you have fewer books you will use the library more. Using the library more means a) they get more funding, and b) you get out into your community and interact with people. (b) does not happen if you stay at home surrounded by your self-created intellectual world. Oh yeah, and you won't be encouraging companies to cut down trees, won't be contributing to environmental damage, and will save money.
Does it get any better than that? Sort through your library; if you haven't read a book in the last five years, get rid of it! If you haven't read it in the past year, seriously consider getting rid of it. Donate it to a thrift store, donate it to the library (most libraries periodically have used book sales where the proceeds go to funding the library), sell it on amazon or ebay (you'd be amazed how much you might get for your books), give them to friends to read on the condition that they pass them on to someone else. But do something! Don't hold on to books that you'll never read again; they serve no purpose on your shelf and someone else might really enjoy them.