There are two general schools of thought on which books to keep, as I learned once I began swapping stories with friends and acquaintances. The first views the bookshelf as a self-portrait, a reflection of the owner’s intellect, imagination, taste and accomplishments. ... Books become stand-ins for friends and clients.There's a very nice piece by Laura Miller in the New York Times on our relationship to our bookshelves and how and when we "cull" our books.
The other approach views a book collection less as a testimony to the past than as a repository for the future; it’s where you put the books you intend to read. ... At its most pragmatic, and with the aid of technology, this attitude can be breathtakingly ruthless.
Of course, I'm mostly in the first category. My books are my old friends. They may be a little tatty and a little age-spotty ... but then so am I. And each title reminds me of the time when I read it, and how I came by it. Every time I've tried to cull books it seems, I end up trying to find copies of the same books I gave away! Hopeless!
But given the sheer number of unread books I'm accumulating, my shelves also serve as my reading list for quite some time to come.