Jessica asks an important question:
why do you hang on to books? I try to give the good ones to friends and if i cant send em off i try to give em to libraries. i never understood that, they get so dusty and cumbersome and if i want to reread them they are in the library… i remember you commeted to me once about how you owned way more books to make some point about how you were much more literate than me.
I certainly hope that I wasn’t using the size of my book collection to make some sort of claim to superiority, but if I did or it seemed I did, I apologize.
One might also ask : why have more than two pairs of shoes? You can just wear one pair over and over until they fall apart. It’s of course a different scenario, but (1) it is more convenient to have more than two pairs of shoes and (2) it brings one aesthetic pleasure to be able to dress in different ways. Having hundreds of books is then the wardrobe equivalent of being Imelda Marcos, but the fundamental two points remain — keeping books is convenient and pleasurable (to me at least).
I do lend books to people, and sometimes I just give them outright, in an indefinite loan, or tell them to lend them to others. Sometimes I have a book for years (Civilwarland in Bad Decline comes to mind) before giving it to someone. However, some books I have are relatively hard to find in libraries or so popular that they are invariably checked out. If I donated all of my books to the local public library or the university library, perhaps 10-15 of them would make it into circulation (reference material mostly). Almost all of them would go up for sale in order to raise funds for the library, which is a good cause, but hardly the most desirable result, which is to give that particular copy of the book a wide readership via library circulation.
That being said, I have used BookCrossing and other systems, and I do prune my book collection by selling things back to used bookstores or donating to the library. But I think there are reasons to have books beyond avarice. The only time I find them dusty and cumbersome is when I move — which I am doing this month, so perhaps I will change my tune then (EDIT: no, I haven’t found a place yet, and I think I am going to go insane).
An Ergodic Walk 
Most of the books I own, I have because they’re either ones I want to re-read, or ones I like to be able to lend. There are probably a few back at my parents’ house that I kept for sentimental reasons, or that are otherwise valuable.
Spending a year and a half being unsettled and mostly nomadic after leaving Boston made it much more convenient to have fewer possessions. 🙂
I need to get rid of a bunch of books before we move. Most of the ones I keep are:
a) Academic/reference type books, which will someday live in my office when I actually have one, or even a cubicle. These are important to have on hand on short notice, and sometimes hard to find in libraries.
b) Books I strongly expect to reread or want to reference when I read later books in the series. Yes, you can always check a book out of the library to reread it, but when you’re looking for something to read *now*, and the library is either closed or not in your apartment, that doesn’t work.
Plus, that “want to reference” thing is pretty important too… For instance, I won’t get rid of a single one of my Stephen King books, because I’m too interested in figuring out all the connections. I hesitate to get rid of Darwin’s Radio, because if I read the next one in that series I might want to refer back to it to remind me of plot points… But will I really ever read the next one in the series?
This is probably the biggest source of kept books, more than wanting to fully reread that particular book.
c) books I’m intending to read but haven’t gotten around to yet. I have too many of those. Perhaps it’s time to give up on a few of them.
d) plays. Which you just never know when those might come in handy. If, y’know, I ever do theater again. Which will hopefully be soon after moving to Ann Arbor, if all goes well.
Two other reasons:
*If people can collect cards, girlfriends/boyfriends, cds, why not books? You might fall in love with one, or many…
*It inspires your friends (like me) to read something when they see all those hanging in (your) bookshelf.