In another forum, I was asked:
You have a less "books=sacred objects" view than many of the people we know in common. Would you say your view is common among librarians of tour acquaintance, or are you an outlier there (too)? Have you always felt that way about books or did you come to it along the way?
Excellent question. To be clear, I believe that individual books can be sacred objects - important/rare editions, religious texts, original manuscripts, individual inscriptions, etc. - but that the format of any physical manifestation of ideas isn't sacred in and of itself. Books are ultimately just collections of glue and paper and cloth; it's the concepts they hold or the meaning we imbue them with that can make them sacred.
In particular, I believe that everyday books are meant to be engaged with, interacted with and responded to. For someone who's a tactile and kinetic learner, this means that I have to write out my thoughts and responses for that engagement to happen; the most convenient, immediate and relevant place to do that is in the text itself. I write in books all the time, and prefer to own the books that really speak to me so I can do so without guilt. I've always worked this way, back into middle school; in college, I preferred to buy the most written-in, highlighted books I could find to continue the conversation the previous owner(s) had started. Marginalia fascinates me, and its place in history is vital. Writing in library books does have historical precedent, too, but I'm less okay with that due a strong "if it doesn't belong to you, you don't get to permanently change it" ethic.
Amongst librarians, there are far more folks in the "books are just paper" camp than you'd think. I'm certainly not an outlier there. Not everyone is as enthusiastic about it as I am, but most of us have to recognize that fact for the very practical reason that we cannot house all of the books in the world forever. Libraries are not warehouses and not all libraries are even archives or research collections. Each individual title that comes onto our shelves gets chosen for its relevance and usefulness to our patrons. Periodically, we review the evidence of that continued relevance and usefulness - number of total check-outs, most recent check-out, date of publication, wear and tear - and when it's become obvious that something's no longer useful, it needs to go, to make room for something that is. Even archivists don't keep everything (ask my friend at the Congregational Library Archive); librarians of all stripes use their best judgment to determine what stays and what goes as a collection changes over time.
When librarians choose to get rid of an item, we do try and see if it can be useful to someone else somewhere, either by relocating the item to another location or by selling it. Then, when it's falling apart beyond repair, or when mold or bugs or water or scratches have damaged it beyond use, the item gets recycled or trashed. Like any other object in our lives, books and DVDs and CDs can carry more negative weight than positive weight; when that happens, it's time for them to go in the most environmentally sound way possible.
All that said, it's occasionally fun for me to watch patrons squirm when I suggest that the paperbacks they've carefully stored in a New England fieldstone basement for the past 30 years are best off destined for the recycling box or trash barrel. Sure, some collector somewhere might want them and they might have some historical value....but they could also give everyone who touches them an upper respiratory illness or contact dermititis. People are more important than books, always. Not necessarily ideas (V for Vendetta and Farenheit 451), but always more important than the physical paper object.
After writing all this, I found a deeply practical (if occasionally defensive) article on What Books You Could Live Without in the NY Times. Read through it all, and the comments below, for some specific criteria in what might stay and what might go as you weed your own collections.