Matthew Cheney is trying to answer this question in the face of a move far away to smaller digs. It brings back so many memories for me of past moves -some to bigger spaces which I have bought book cases to fill and some to smaller spaces where I've been forced to make difficult decisions in answer to that very question: "What are you going to do with all those books?"
I've considered the value of keeping only The Classics, no matter how battered and even if the books in question are paperback ex-lib editions with underlined pages and missing covers. I've considered the option of keeping only those books that held sentimental value to me, like the copy of Michener's Hawaii that brings back that high school summer vacation like it was yesterday.
Both of those choices have since been rejected, and all of the books that fit those categories are gone.
But I still have more books than I have room for and a hard time culling the flock. I am at a point, with our youngest kiddo in high school, where I don't have to save all those homeschooling materials for the next child. This past year was our last trek through Ancient History, so I can pass along the old ex-lib editions of Greek plays, those falling-apart Viking Portable Reader paperbacks and other books which I don't foresee re-reading. But even so, I'm having trouble. Why do I want to keep books that I will most likely never re-read? Some are reference books of material not available on-line, so that's a good excuse. I love a good excuse.
Now that I've been trying to get to know mysteries I have a growing collection of (mostly) paperback mystery novels. They are taking up a growing amount of shelf space. I got rid of some of the science fiction books, but, even so, the amount of space needed for my sff books increases yearly.
It's easy on the one hand, because I'm not having to pack them up and move them.... Still, I can't help but ask myself the same question that Matthew Cheney has answered for himself: "What are you going to do with all those books?"