I try and keep all my books but the piles that overspill from shelves annoys the wife and so I've routinely culled my collection. Rather than try and sell them (with the associated knowledge that you've been conned that we all experience) I box them up and give them to local charity shops. But there are more and more books that I won't get rid of, whether it's because I'm becoming more selective in what I buy, or I'm deluding myself into thinking that the book will eventually get read/re-read